Don't listen to the person above. I have Verizon with unlimited messages. You can use the built-in AIM messenger on your phone, or have AIM send your messages via text to your phone. Either way it is included. Trust me, I have been using this plan and am on AIM on my phone very frequently, I have never been charged other than my original monthly plan!
Actually.. you DO get unlimited IMs when you have a unlimited messaging plan. I recently got Verizon, and on the receipt it says IMs is part of it. and I've sent MANY ims and have never gotten charged, so as for the answer "it's a different plan".. research your answer BEFORE you answer.