It may b da cables. I recently hooked my wii up to a Hdtv with 3rd party cables it didnt look right either. Contrary to popular belief, the Wii is capable of awesome graphics!
I would say its just da cables or something is just not set right with the HDTV.
Also b4 I get creamed for saying the Wii has awesome graphics, Not all current games will prove dat so far.
But if you check out Iwata ask on Wii.com, the developers and creators of the Wii will change your mind about what the Wii can and cant do.