Having heroically used “Search” to find that there was actually an existing suggestion along these lines, I just want to resurrect this “mad, but not entirely insane” (I think) idea.
Yeah, the killer is that Scrivener would have to be entirely rewritten. But, then again, considering that nowadays (late 2010) the Scrivener for Windows project is “a completely separate development effort” anyway, perhaps a completely separate “Scrivener Web” development effort would not seem so crazy. (Still crazy, perhaps – just not so crazy! 
)
I was thinking about this today – immediately after upgrading Scrivener on my Intel Mac Mini running 10.6 to Scrivener 2.0 and while wondering whether it was possible and (if possible) useful to getting the upgrade for a copy on my aging G4 iBook running 10.4. I think it would work, but I don’t use the (slowly dying) laptop that much anymore, and might well never replace it, so I’m not sure it would be worth it. Nevertheless, for those times when I am bouncing around and trying to write on the move, I am seldom sure what kind of computer I’ll have access, too – and so I thought, “A Web version of Scrivener that could access my Scrivener project files either from a USB Flash drive or from online storage would be pretty cool, actually.”
Now while I am not (currently) much of a cloud-computing road warrior, I am aware that there are trends towards more of this kind of thing for both apps access and data storage. So while it might not be an immediate hit to roll out Scrivener Web tomorrow, it might nevertheless seem quite timely within the foreseeable future of the next year or few.
Likewise, I read a lot about ongoing trends towards the growing significance of of social-network-like functionality for various kinds of online businesses, as well as collaborative work, and while the “writer’s art” might continue to be perceived as solitary, there would nevertheless be – besides possibilities for building features for collaboration and content sharing into a “Scrivener Web” – opportunities to harness an online Scrivener for, say, the development of online writing groups/workshops (not to mention for use in universities and other academic environments).
In such contexts, I think Scrivener’s underlying philosophy of being not just a kind of word processor but an integrated “project management tool” would actually put it in a strong position to achieve widespread use as an “online writing environment” for many kinds of writers.
So, though the idea of the work involved for anyone developing “Scrivener Web” might be horrendous 
it might nevertheless be an idea worth keeping in mind. An idea whose time, if not yet actually come, might nevertheless be coming.
Cheers,
Carl