Encrypt files with my own private key

Mozy allows the user to use his own encryption key, but Dropbox does not.
The workaround is to use encrypted files/volumes (eg: Truecrypt, encrypted dmg) on my computer, but this solution is more complicated to use.

On this subject, Dropbox's founder Drew Houston said an interview given in March 2008: "We are working on a way for people to provide their own private keys or passphrases for people who choose to do so as long as we can preserve the simplicity of the user experience."

However, I haven't seen anything new since. Maybe it is harder when you use binary diff synchronization?