I was doing it with a couple of my domains. It worked very smoothly. I'm going to be doing the same with VideoSlacker.com really soon. The only thing, though, is that you don't get the option to advertise in the emails anywhere and you have to put a link to the actual login. For instance, my site was Reality TV Friends.com . I had to put a link to
http://mail.google.com/a/realitytvfriends.com/ in order for my peeps to login. It breaks out of frames, too.
To signup, go to
http://www.google.com/a/