This concerns using any sort of technology which promises to keep local copies of your work synchronised between multiple computers over the Internet, such as MobileMe (iDisk), DropBox, WebDAV based technologies, and so forth.
Due to the way Scrivener works with a project file, and the manner in which most of these technologies keep themselves up to date, it is not recommended that you use these methods to work on active projects. Unlike many file formats, Scrivener projects are actually composed of many small files (sometimes this can range in the hundreds), which depending on how you work, are updated nearly continuously as you arrange, write, edit, and sort the material in your book. This level of complexity in combination with the number of network connexions required to keep everything up to date can dramatically increase the probability of suffering lost work, and corrupted projects.
It is possible to reduce the probability of this happening by adjusting how often Scrivener automatically saves your project, and as such if you choose to use Scrivener to edit projects that are located on these network services, you can probably reduce your risk by changing that setting to something closer to a minute or greater in length.
Despite, I still recommend that you keep your working projects on local hard drives (or external drives that are plugged in via USB or FireWire), and use these network services to store your backups. Scrivener makes it very easy to make backups, and you should be doing this as you work anyway. I highly recommend using the zip archive option so as to make sure that the many small files in the project do not get lost in the flurry of network activity that would otherwise result in saving a Scrivener project to a network drive.
To summarise: Use of Scrivener with network synchronised folders and mountable drives is not recommended and you do so at your own risk. The recommended use is to use Scrivener’s back-up feature (found in the File menu) to produce zipped archives which can be safely saved to these drives and available to all of your computers.
Work local; back-up often; and develop habits for backing up your last copy of the day so that you’ll always have the latest version available to all of your networked computers.