I used to do this for passwords and such. The problem was that the file would become un-synched and I'd end up with several "<computer name>'s version..." copies. The problem is that with Dropbox you aren't accessing a single file, you're accessing a local copy. So concurrent access, especially when a file is being held open on Windows, creates problems.

Because dropbox will save bandwidth by only updating the parts of files that have changed. If you use truecrypt the entire file will change for every small change you do. Basically you lose any advantage of using dropbox over a standard FTP system.