Well, if it were to use email, then the sender would have to give an email address, so it won't be anonymous anymore.
I am planing on making a message center for the site over the holidays. This is a web based messaging system that is not email but looks kind of like it (check out the Message Center on MTV.com). I suppose it wouldn't be too hard to add the capability for anonymous messages. We would just need a way to tell the anonymous user's apart. I could probably use a randomly generated ID.
There is a pretty good chance I can get this going. I will have to think about it some more.
-Mike