I suppose it's worth mentioning that I've been experiencing a set of problems with my N900 and T-Mobile (though not to the degree mentioned in this post). In my case, my N900 thinks it has no SIM card in it, and will not connect to T-Mobile. After about five minutes it figures out that my SIM card is actually in the device, and connects - after which point there are no issues.

I did a complete reflash (both EMMC and combined image) and the issue persists. When I asked T-Mobile what they could do about the situation, they said they were "Updating the towers" and told me to come back in a few weeks.