It supports both. There is only one model of the N900 (RX-51) that has been circulating among the press and it passed the FCC a little while ago.

Here's the FCC report:
https://fjallfoss.fcc.gov/prod/oet/f...ive_or_pdf=pdf

Scroll to page 3 and you'll very clearly see "WCDMA 1700/2100". In other words, *you will get 3G on T-Mobile US as long as you're in an area with 3G coverage*.
To further confirm this, a HoFo (Howard Forums) member recently visited the NYC Nokia Flagship store and tested out the N900 with his own T-Mobile sim card. The 3G symbol showed up as expected. I believe he even put up a picture showing this.

I really don't get why there's so much confusion over this. T-Mobile US customers are set. It's us AT&T customers that are out of luck. All we can do is hope that Nokia will release a device relatively soon with 3G support for our network.