There are currently 4 poker apps for iPhone and iPads. Bwin is the only native poker app that has been approved by Apple and is listed in the iTunes App Store. Switch Poker, mFortune and Terminal Poker offer web apps. They all work pretty well although there are pretty big differences in the quality of the graphics. You can see screengrabs of the all the games on http://www.iphonepoker.net/ which might make the decision easier.
None of the apps I've listed accept players from the United States. Full Tilt did briefly offer an iPhone and iPad poker app before they lost their license.