T-Mobile generally won't care if you tether your device (if you can at all), it's just that you wouldn't get much use out of it because as with all other "unlimited plans" your real cap is just 5 GB a month. So outside of any browsing or work, if you plan on using it for videos or downloading any files your going hit the cap real quick and be subject to slow downs for the rest of the billing cycle.

Right now the phone I got from TMobile (Nokia 5130 Music Express) can tether to my n800 tablet and that's what I've been using so far. And the n900 will be able to tether to PCs too.