As Wired and IGN have discovered, the Wii looks significantly better using component cables with an HDTV versus its bundled composite connection. 480p is a big step up from 480i.
HDTVs look great with HD sources, but when playing analog signals, they look even worse than the analog TVs they replace. That's one of the dirty secrets of the HD upgrade; 480i games and broadcasts look weak. If gaming on a TV with component inputs, those cables are a must; they even make Xbox, PS2, and GameCube titles clearer. (And for fully HD consoles, cables that carry digital signals -- like HDMI -- are best.)