A pixel on iOS is the full resolution of the device, which means if I have an image that is 100×100 pixels in length, then the phone will render it 100×100 pixels on a standard non-retina device. However, because newer iPhones have a quadrupled pixel density, that same image will render at 100×100 pixels, but look half that size. The iOS engineers solved this a long time ago (way back in OS X with Quartz) when they introduced Core Graphics’ point system. A point is a standard length equivalent to 1×1 pixels on a non-retina device, and 2×2 pixels on a retina device. That way, your 100×100 image will render twice the size on a retina device and basically normalize what the user sees.
It also provides a standard system of measurement on iOS devices because no matter how the pixel density changes, there have always been 320×480 points on an iPhone screen and 768×1024 points on an iPad screen.*
But at the same time, you can basically disregard the documentation considering that retina devices were introduced with iOS 4 at a minimum, and I don’t know of too many people still running iOS 3 on a newer iPhone. But if such a case arises, your UIImage would need to be rendered at exactly twice its dimensions in pixels on a retina iPhone to make up for the pixel density difference.
*Starting with the iPhone 5, the iPhone’s dimensions are now no longer standardized. Please use the appropriate APIs to retrieve the screen’s dimensions or use layout constraints.