Sync your photos for both an older iPhone/iPod touch and iPhone 4. Keep it at the same brightness. Mine was at 50% default with auto-brightness off. Make sure the photos were taken at higher the iP4's screen resolution of 960x640 (like 2-3MP). I have a photo album at VGA, and it doesn't show quite the same details when I zoom it to the max. And then pinch it all the way. No pixelation at full zoom. Comparisons between the old vs new is like night and day.
It makes me want to sell my iPod touch right away or throw it against the wall like a caveman because on how prehistoric it looks now overnight. Even my laptop computer I have the urge to sell. I can never go back to those crappier looking screens again. The Retina Display enhances your picture viewing experience. You would ask yourself, "Who knew my camera took such great photos?" Because even some people's computer screens when they uploaded them on their hard drive looks like crap with its lack of pixel density.
The Retina Display is the most expensive part of the iPhone 4. Well worth the admission.