It's not that absurd.
Mainly because one might overuse the expected amount of data that needs to be used. This puts pressure on a network's resources. Text isn't the same as data. Data takes a lot of resources and is expensive. Remember, unlimited data plans is an American thing. In Europe, you pay by the gig.
But, I wouldn't be suprised if T-mo didn't block it on the N1. The N1 isn't a very popular phone (not many bought it) and not many people use T-Mo's 3G so I don't think it would be worth T-mo's trouble to try and milk customers. (or worse, make them unhappy) One of T-Mo's assets is its customer loyalty. Hopefully, they will be wise not to mess with it.
I mean, why pay (or limit) the N1, when EVO is knocking on my door asking to play?