Yes, for truely unlimited 4G data it would be an extra $20/month. But if your phone only does 3G, the people at T-Mobile assured me that once I exceed the 500 megs of 4G included in the basic plan, that my 4G speed would be throttled to 3G (instead of 2G like it says in some places).

Since the N900 is only 3G anyway, I'm happy with that.

We'll see if they were actually right.