You can do AOL messaging at a special WAP website, but you have to keep hitting refresh to receive new messages.
Alternatively, you can obtain an inexpensive MDS hosting service such as http://www.myblackberry.us
and then get instant messaging working.
Or wait until around November/December when TMobile releases BlackBerryOS 4.0 ... which should permit your BlackBerry to run direct IM software without need for BES/MDS. (Or get MDS hosting till then)