I have an existing 'unlimited internet' account with T-Mobile US for a long history of WinMo devices.

When I got my N900 last week, I put my T-Mobile SIM chip in it and the phone connected at first-boot to the T-Mobile network automatically (via the internet2.voicestream.com APN). The status bar indicator changes between "2G", "3G", and "3.5G" as I've moved around Seattle. Speed tests suggest I'm getting .35MBPS download and .75MBPS upload.

The speedtest results appear the same after switching the APN to epc.tmobile.com.

Your mileage may vary but there's hope on the horizon: T-Mobile Rolling Out HSPA 7.2 Before Years End