At present time, there are only a couple of unlocked smartphones that work on T-mobile USA 3G.

1. Pharos 137 - WM platform, I hear that it is still a WIP, but it also sounds like Pharos is very responsive about working with their customers on bugs and providing firmware updates

2. Samsung Galaxy i7500 - Android platform. Rumored to be subsidized by T-mobile eventually.

3. Unlocked versions of the G1 and MyTouch apparently work

4. T-mobile itself subsidizes several smartphones

I don't think any of these are really comparable to the N900 in terms of specs, and of course the N900 isn't really a smartphone. What didn't you like about the N900 other than the defect? I really am enjoying using mine.