If you're worried about privacy on your Gmail account, or you need to exchange encrypted e-mail on-the-go, there's a new Firefox extension that may be just the ticket.
Gmail S/MIME (from the same person who came up with the Gmail Filesystem last year) can send and receive encrypted e-mail in the same format that normal clients like Mail and Thunderbird use.
So, if you like the Gmail interface but want a bit more security, I'd recommend giving this a try.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20050422014947600