The problem is to get the simplicity of Dropbox (the filesystem is the UI) into your encryption system.
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.
Neither is TrueCrypt. I'd love to use it with Dropbox on my Android phone, but that's just not possible. I use EncFS on Dropbox for certain files that I know I will not need to access on my phone. That's an annoying compromise though. I'd much rather encfs the entire thing.
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.