The solution is quite simple: use Dropbox to synchronize a single binary file that is a LUKS-encrypted Linux filesystem (or TrueCrypt or whatever).

This meets all the requirements this person is asking for. All his systems could even be mounting the image using different passphrases (IIRC LUKS allows up to 8 different passphrase for the same image). It will work well because Dropbox will of course synchronize the deltas and not transmit the whole image every time a single byte changes.

Only downside is you can only have 1 client mounting the image at a time... Perhaps some clustered filesystem could get rid of this limitation (that would be an interesting use of them).