I use this setup too, and I really like it, although I use Daedalus Touch and not Notebooks. All told, the iPad with bluetooth keyboard & origami case is still a bit lighter than the MacBook, plus you have more flexibility when you pack it in a backpack (I tend to pack roughly 10 books with me when I write, so this is nice). I also don’t agree with the sentiment that if you are going to get a keyboard you might as well get a MacBook Air. While the Air is obviously more powerful, the iPad allows you to have other capabilities of interacting with the data, ways that I find more interesting and “hands-on”. You can pick it up without the keyboard. You can interact with what you are reading in a more “natural” way: through touch. For example, I grade student papers using the iPad, and the experience is more like holding a real paper copy (a copy I can scribble all over, with arrows and lines) which I personally find makes the (already laborious) grading process more palatable. 
And when it comes to the writing experience, I do enjoy using the iPad–it is a different experience than writing on my MacBook. I find the limitations on the iPad actually to be beneficial to the writing process. Sometimes the extra capabilities of the MacBook or MacBook Air can get in the way (distractions distractions). But the one limitation to serious writing on the iPad is the glass keyboard. Add a full-sized bluetooth keyboard to the mix, and off you go! 
I’d add two other limitations on serious writing on the iPad, at least for me: you can’t display two documents side by side, and you can’t use Scrivener. That’s one of the reasons I wound up getting a 13" MBA last year instead of using a 50% cheaper iPad (which I already had) + Mac Mini combo to replace my old MBA.
Now I plug my MBA into my big external display most of the time, and use the iPad for reading, note taking, etc. But if I’m away from home, I want to use Scrivener and display my notes and story in progress side by side, and that means I need more than my iPad, even though a friend gave me an extra apple bluetooth keyboard to use with it.
Still, Michael, if you’re replacing your desktop (iMac or Mini) and will do most of your work there, I do think the iPad with BT keyboard is a viable writing platform, as long as you don’t mind looking at only one doc at a time. It’s not as fast or efficient as using a Mac because of the limitations of iOS, the need to touch the screen instead of using a trackpad, and so on. But if you’re mostly taking notes or just writing on a single document without frequently switching to others for reference, it should be fine, and the battery life and light weight are appealing. But I tend to do more than that on the road, so for me, the iPad wasn’t sufficient.
But even though I’m not quite bold enough to go laptop-less yet – the MBA still seems like the perfect machine to me – that might change depending on what happens with Scrivener for iOS, with the development of iOS itself and the iPad, and how much writing I wind up doing at home. Even now, I use my MBA only with the big external display more than 90% of the time. When I do anything except write, I’m generally using the iPad. But I’m a professional writer, so that means I’m writing a lot, and if I’m doing it away from my house, at this point, I want a laptop.
Also, Michael, you may be surprised at how much of a qualitative difference the lighter weight of the MBA makes vs. your MacBook. It just feels so much easier to grab and go than any other laptop I’ve ever had, so I tend to take it more places.
Anyway, I’d certainly wait to see what happens with both the MBA and the iPad if you can afford to do so for another few months or a year. Good luck!
Soon you’ll be able to turn your iPad into a Macbook Air and back again.
http://www.kickstarter.com/projects/552506690/brydge-ipad-do-more
This is really where Apple should be heading, except with one crucial difference: when the screen is plugged into a keyboard it becomes a computer instead of being still being a mobile OS intended to be held in your hand. Screen tapping as the primary and only way to interface with objects interactively is no good in this layout. As a supplemental method of interfacing—sure why not, the technology is already in the screen. Anyway, it should be a Mac when you have it plugged into a docking station or portable keyboard, and a touch mobile OS when you’re carrying it around. In the future I suspect we’ll all be carrying our computers around with us in our pockets and plugging them into workstations when we arrive. That’s why this isn’t really a “MacBook Air” as you put it. You can’t run Photoshop, Scrivener and DEVONthink Pro on there, but I can with the same form factor and an MBA. The iPad needs to actually become an MBA when in the keyboard for me to really take interest.
Or to put it another way, if I could detach the screen from my MBA and use it like an iPad, that would be really cool. 
I’d like to see it morph toward a cross between the all-in-one iMac and the old Mac Duo, where I’d feed the iPad to the iMac like bread into a toaster, and it magically becomes a for-real computer, and I could moil away through the day in Scrivener and InCopy and all those sorts of bills-paying things.
And then when day is done I could press the button, and out the iPad slides, and I could go sprawl on the couch and watch Pan’s Labyrinth and suddenly remember what I was trying to say in this morning’s chapter, and hit Pause and finger-up Scrivener and make my corrections in the live file with the iPad’s on-screen keyboard, and then realize I need to say more and grab the wireless keyboard and start right in.
One can dream.
Sounds like a good one.
the multi core A5 isn’t nearly as capable as we think. Yes it’s powerful. Yes it can do a lot. But as OSs mature they require ever increasing horse power (not quite Moore’s law but…). You could run OSX very slowly on an A5 or you could run it a bit faster for a short time (battery life would dramatically decrease as the whole chip would be needed effectively disabling the low power operating mode).
I would like to see the iPad mature into something like Ahab suggested but with a slightly larger form factor and a “real” processor. A docking station would provide port replication and full facilities. In undocked mode it would provide an optimized UI or a full UI.
The other alternative that I feel is very bad would be a portable X-window display. The iPad is just another IO device to your desktop. Think logmein or those other desk top viewing apps.
Although my first computer used punched cards and came equipped with orbiting satellites of unsmiling crewcut operatives wearing Univac nametags, I claim, after 40something years as a “user,” near-zero functional knowledge of the things. But when I was fantasizing about my dream computer, I rather thought of the iPad as being the iPad, with its lightweight operating system and Suzy-Homemaker Chipset, and the real computer, with its glass-pak mufflers and fuel injection and bumpy camshaft, living inside the monitor, where it bides its time awaiting the insertion of its little brother, and then assumes full control upon touchdown.
The MacBook Duo, if the spot in my brain where my memory used to be can be trusted, outsourced much of its graphics, i/o, and portagery into its docking station, allowing for what was, at the time, the tiniest useful computer we ever did see. “Who’s a clever little computer,” the lucky owners would say. “You are. Yes you are.”
You are correct. The docking station was effectively an extension to the hardware which allowed for much improved performance. The real problem is the A5 (multi-core or not) simply isn’t up to the task of a full system. That and that iPad doesn’t currently have a port capable of sending raw data to external video processors (meaning the on board vid-proc still does all that work even when an external display adapter is connected).
What you describe would be possible but would likely wind up with a larger foot print. Something closer to a MBA sized device running i5 or Atom processor (A6?) with a real IO interface at the bus level (thunderbolt could be a first step in that direction). If they build something like that, I would be a poorer person for a bit.
Not that I have thought of this much…
A late reply, but I wanted to add that I have and use the Incase Origami keyboard cover / iPad stand. It’s light and easy to set up, the only caveat is that you do, indeed, need to use the Apple BT keyboard.
There’s also the Amazon Basics BT keyboard for around $30 (the price wobbles quite a bit… the highest I saw it at was $43+, but I put it on my Wish List and waited). It’s a tad smaller than Apple’s BT keyboard, but it’s light and has lighted keys and pairs quickly with my new iPad. It has a physical on/off switch, and takes two AAA batteries (included).
It’s light enough and cheap enough to slip into a cover and head out without worries.
Hope you’ve been enjoying whatever choice you made.