Right now you get the Angry Birds HD App via the iTunes store for FREE (a $2.99 value). This is compatible with iPad. Requires iOS 4.3 or later.
You can also download Angry Birds Space (non HD) for FREE for your iPhone 3GS, iPhone 4, iPhone 4S, iPhone 5, iPod touch (3rd generation), iPod touch (4th generation), iPod touch (5th generation) and iPad. Requires iOS 4.3 or later. This app is optimized for iPhone 5.