I am not sure if I am the first to notice this but there is a new link on the BlackBerry Help page at mobile.blackberry.com. It now lets you search to see what instant messaging applications are available for your BlackBerry.
Just go to www.blackberry.com/instantmessaging/ from your BlackBerry!
You then click the search button and it will tell you what instant messenger applications supported by your BlackBerry.
Once I resent my service books AT&T is showing: Google Talk, Windows Live Messenger, & Yahoo! Messenger!!!