Soon after I upgraded to Leopard I also switch Mail.app to use IMAP. I had been using Entourage but I wanted to check out the new Mail.app features. I had not noticed at first but when using Mail.app to reply to a message the drafts would get saved to the server, so when you looked at the thread in gmail, it would be this long thread with mutiple half assed messages. I thought I sent the person I was responding to 12 e-mails. Turns out this is not the case, the final one gets sent when you click send. It just seems to muck up the web interface, you can read the thread about it on google groups. You can fix it by going into mail prefs and turning off saving drafts on server.