UPDATE:
Syler has a good point in reminding me that the new "retina" screen resolution of the iPhone is 960x640 pixels, and not 320 x 480 anymore.

So this is the resolution to deliver for the iPhone version, we'll make another version for legacy iPhones (99.5% of the market) by downsizing the resolution).

Keep in mind that the iPhone screen size didn't change, it is still 3.5"!