I talked to T-Mobile about this before I ordered an unlocked N900, just to be sure. I have a G1 right now on the $25 Android unlimited data + 400 message plan. I was told all I have to do is call them back when I get the phone and they will switch me to a special data plan for unlocked phones. A simple click-click re-configuration on their end of some sort and I'm good to go. No waiting for it to take effect. No change in price or service, still unlimited data + 400 messages. So apparently no problem, but well find out for certain when that N900 arrives.