Encryption seems like a much better solution to this problem than reinventing the entire Dropbox wheel.
Sure, you can just put a Truecrypt disk image in a Dropbox folder, but you're missing the granularity of Dropbox (e.g. to sync certain files to my SSD) and that granularity also affords you protection against race conditions (if both machines have the encrypted drive mounted and are writing, sounds like trouble).
Not an insolvable problem by any means, but not one that has yet been solved either.
Unfortunately the one time I installed it the user experience and interface was miserable..