Why wouldn't an iPhone be able to use this plan? I know a lot of unlocked iPhones using Tmobile already, and isn't ATT about to lose their exclusive rights to it anyway?

I think this would (if true) be a bigger game changer for Tmobile than Nokia. Now, if they had come out with this n900 + 50 all you can eat plan earlier in the iPhone's life they could have seriously put a dent in the market.