Update from RIM (4:00p ET):
RIM had this to say. At this point it seems more like a Tmo issue than a RIM issue. We'll update this if we find out more.
We are aware that some users on the T-Mobile USA network have reported a service issue and we are currently investigating the issue with T-Mobile. BlackBerry customers on other networks in the United States and other countries are unaffected.
Reports are rolling in today that T-Mobile BlackBerry users may be having some trouble accessing email, BlackBerry Messenger, and other BIS services. Several CrackBerry members have spoken with customer service at T-Mobile and have been told various things (it's T-Mobile's issue, it's RIM's fault, it's an issue with authentication servers, it will be restored in 2 hours, it will be restored at 10pm tonight, etc). If you are experiencing issues as well, leave a comment below or head on over to the forums to discuss.