JMFO,
After you make certain that your APN is set to wap.voicestream.com (Settings/TCP) you may need to go to your BWC page (
http://www.t-mobile.com/bwc/), login, look at your 'Profile' and send the Blackberry Web Client Service Book again. I deleted my old one before I had the new one sent but I don't know if that's necessary. If you decide to delete it, it can be accessed through Settings/Service Book. You are looking for Web Client [CMME]. Highlight it, press the thumbwheel and choose Delete.
It got mine working. I have a 7100t and OS 4.0.
Marshall