Maybe the key is unlocked? I suppose the Google ION was unlocked too though, so most likely this will have the same fate. It's possible that t-zones could work up until T-mobile starts selling the phone in their stores.
To the best of my knowledge the whole blocking thing is done via IMEI numbers. Maybe T-mobile got the whole range of myTouch numbers loaded into the system when they started selling it, and the google ION was covered in that range.
At this point, the fate of the Nexus One is completely in the air given its status as an unlocked phone, IMO. Even if T-zones works now, it might stop working in a few months. I would count on having to buy android data eventually.