T-mobile's BBs come with OZ instant messaging which offers AIM, Yahoo, ICQ, and MSN they talk over your SMS and not data so use appropriately...jive talk seems to be the drink of choice around these parts so at least you know there will be plenty of people able to help you out with any problems you have...hope all this helps
If you'd rather not use OZ instant messaging you could just set up your IM from a PC so that it forwards all your messages to your mobile device. The messages it sends are standard text and allow you to reply to them...you don't even need to run the other program...up to you...