It's hard to say. Tipster Roberto says he's been in touch with T-Mobile, and got a reassuring, if not particularly explanatory, response:
I just got off the phone with T-Mobile who transferred me over to HTC Technical Support because I was not receiving data on my phone, opening the Android Browser indicated I needed to contact T-Mobile and add a DataConnect Plan in order to receive data on my phone.
T-Mobile check everything on my account & appeared ok then transferred me to HTC Technical Support who indicated there currently is a problem with most Nexus One devices connecting to Data Services on the T-Mobile Network and they hope to have it fixed sometime this morning.
Frustrating to say the least!
This seems to be the common experience: The prompt to buy a new data plan, the call to T-Mobile, the transfer to HTC support, then the claim that everything will be fixed soon. Some users are reporting HTC reps saying all HTC devices are having issues on T-Mobile, but the evidence—the growing thread of reports from users—doesn't seem to bear that out. This looks like a Nexus One problem, and a strange one: Without a carrier fault or software update, why would a phone just stop working?
I'll let you know when the dust settles—throw your theories in the comments. —Thanks, tipsters!
UPDATE: The issue has been fixed. Nexus One owners were the only ones affected, though the blame seems to fall at T-Mobile's feet. To anyone still having issues: just restart your