JiveTalk is one of the best paid IM available for the Blackberry today. It's one time fee of $19.99 or $29.99 with a chance of using it on other Blackberry if you ever decide to upgrade your Blackberry.
Now to confuse you even more, T-mobile do offer a plan that combines voice, data, and unlimited text messaging. It's hidden away at T-mobile's website so not too many people know about it. But here's the link...
Plan 1
Plan 2