If they did it for iMessage and Facetime they'd capture all chat traffic in America in a week. They'd risk losing the premium status of the iphone, but I think the real risk is they'd start a war with facebook and google. They'd be betting that google and facebook would keep their apps on the iphone if they invaded the android ecosystem. At least that's what I think. I'm not sure if Apple is ready to say all their applications are better then Google's yet.