Hmm, I wonder if T-mobile is finally cracking down on people using cheap data plans with their unlocked smartphones.

Anyway, technically, you do need a special smartphone plan. However, T-mobile only enforced this restriction on their subsidized smartphones (and their unlocked analogues).

But before you do that, you can try another APN: internet2.voicestream.com works for a lot of people. Other APNs might work too.