A "smartphone" on T-mobile is whatever T-mobile calls a smartphone, whether the designation is grounded in reality or not.

It's apparent at this point that T-mobile can detect your phone's IMEI number. It's just a matter of whether T-mobile has the will to force you on a higher plan knowing that you really do have an unlocked smartphone.

I would guess that the cheaper plans will work for awhile, if the n900 remains unsubsidized. They might "suggest" that you change, if use of the unlocked n900 becomes widespread. The probability that they will force a data plan change if you are on a cheap plan goes to close to 100% if they decide to subsidize the phone, even if you bought unlocked.