Me and bla have been drewling aver possibility of doing this forever. I even tried to tie PGP or GPG to Gmail but it does not support their transferring protocol :( Well, now it's possible.
The encryption is not super tough but it's enough to hide things from an average observer.
Its implemented using a Greasemonkey script (It's a Firefox extension). The only thing that you need to do past that is generate your key pair: public key so that other could encrypt messages for you and private so that you could decrypt messages sent to you by others. It's a fairly easy implementation for it.
My public key is: 1817047:17
Here you can get the actual script.
Send me an email to check, you can also post your public key in the comments for all those that wanna try it out.