Update: As of 12:54PM ET the issue has been fixed, although there hasn’t been an official announcement of what the issue was.
Users of the Google Nexus One on T-Mobile are reporting a nationwide data outage this morning. Users are reporting that their SIM card will work in other phones in the same area. While it appears to be widespread, it doesn’t appear to be affecting every user. We’ll give more information as we find it out. As expected, both HTC and T-Mobile technical support is experiencing extremely high call volume this morning.
Source: Engadget