That's not always true. You are generally in the clear with using a cheaper grandfathered data plan with unlocked devices. But all bets are off once they decide to support a subsidized version of your device.

This happened with a couple of Android phones. People were able to use the cheap plans on the unlocked version of the T-mobile MyTouch for awhile. However, when T-mobile was close to launching the subsidized version, they forced the users of the unlocked version to upgrade to the Android plan, if they weren't already on it.

I kind of doubt that they would subsidize the n900, since they are probably hearing about the WIP nature of the device--there are other N-series devices make better sense to me, like the N97 mini. But if they did, there's a good chance that the $5 & $10 plans will stop working. The $20 "Total Internet" plan might be cut off too (depending on whether they group Maemo with Android), but since it was the grandfathered "smartphone" plan, they will be more likely to let you keep it.