There has been no official word from T-Mobile with information, but their Twitter account acknowledged the outage and that work was being done to clear it up. As I said, the outage isn't complete. I talked (ok, I tweeted) with a friend in Hawaii and he reported that there were no issues there. I suspect that is a totally separate network though from the one on the mainland.
The last few weeks just haven't been that good for T-Mobile. The Sidekick fiasco put T-Mobile in the news in a bad way, even though that wasn't their fault at all. Microsoft owns the Sidekick and was the cause of that outage. This one though lays at T-Mobile's feet. I'd bet more than a few Sidekick owners tonight thought there was another service issue with their devices though.
In addition to the Twitter account for T-Mobile, you can follow a discussion thread on their support forums. As of 9pm eastern, they claim the outage now only affects five percent of their users and they are working diligently to restore service to those accounts now.
It is good to have Twitter to get information about this so quickly. Before Twitter became so popular, many users would suffer through outages like this thinking it may just be their phone or would be on hold for an absurdly long time on a support call. Kudos to T-Mobile for updating its users informally in the support forums and on Twitter. Maybe by Wednesday they will have it all resolved and will let everyone know what the issue was.