NJ5 on 11 December 2008
It depends on how far you're from the TV, as someone's link shows.
If your face is glued to the screen, you can distinguish the pixels and even the subpixels (red/green/blue). If you're far enough, you can't tell 480p from 1080p.
My Mario Kart Wii friend code: 2707-1866-0957