Synchronizing all your files sounds like space-waste. It would be trivial to determine if dropbox does that, simply by monitoring the number of bytes sent to dropbox (Create a new file of size x, determine if the stream to dropbox transmits at least x amount of bytes) or by replacing the SSL certificate in the program with your own and then set up a fake server (This would be harder) to determine exactly what is being transmitted.

If this is indeed something nefarious, I would much rather assume it would transmit file hashes rather than the files itself. Although I can't possibly imagine this is actually true, the implications would be devastating for dropbox and it should be easy to verify by an independent third party.