Just a data point here... I've been using T-Mobile with Chatter for 14 days now (about half a month, IOW), both for IM and for eMail, with 20 contacts on my contact list. I also tend to use more data than would be typical, because I'm frequently restarting the app (which eats more data) as I debug the various versions. I've also used Blazer a bit.

So ... my data usage has been 4.4MB during that time. I think it's fair to say, then, that a "normal" moderate-to-heavy user would put in between 3-8MB per month.