If the iPad 2 gets a Retina display, what will the resolution have to be? Almost any person who is talking about an iPad 2 strongly believes that it will have a Retina display. Apple had enough trouble quadrupling the number of pixels on the iPhone 4.
Original iPhones: 480 x 320 = 153,600 pixels at 165 PPI
iPhone 4: 960 x 640 = 614,400 pixels at 330 PPI