I've been on T-Mobile for 12 years. You'll have no problem getting an unlocked N900 up and running on T-Mobile. T-Mobile has the cheapest plans and are the most friendly US provider toward unlocking their phones and using other unlocked phones.

But some of what longtime users like me could tell you won't apply in your case as a new user because we're grandfathered into old plans and/or services.

T-Mobile.com will pretty much tell you about everything from plans to services to coverage. Then post back here with more specific questions and I or someone should be able to give specific answers.