No, at present you cannot download any betting apps from the iTunes App Store in the United States.  Apple will only approve real money gambling apps in jurisdictions where mobile gambling is legal.  Since online betting is a legal grey area in the United States, Apple have so far refused to allow U.S. iPhone betting apps into its App Store.

You can still place bets from your iPhone by using Web apps rather than Native apps in the United States.  Web apps are browser based so there is no need to download or install any software from the App Store.  All you need to do is browse to the iPhone optimized version of your favourite U.S. friendly sportsbook.