There are no US-facing poker sites that have iPhone apps. FT released Rush on Android only, not iPhone, because it was Flash-based and also I believe Apple doesn't allow real money
gambling apps in their store. Then not long afterwards it was pulled from the Android Market for the same reason (violating their terms against real-money gambling). I don't know if their terms have changed since or only applies to US players, but none of the remaining US rooms have the option that I know of. Besides, playing any serious poker for any length of time on a tiny iPhone screen would make most people go batshit insane.
EDIT: Apparently it's a US restriction (big surprise). OnGame/Bwin released iPhone & Android apps last year. It looks like the only US mobile option aside from Rush was Cake's Windows Mobile app. Considering the near-death of that platform and since BF I'd be surprised if it's still around.