Yeah, my brother rented CoD3 yesterday and the graphics really do help the immersion, I was really impressed.
Unfortunately I don't reckon CoD3 will do that well on Wii... It's just one of those games where you need the graphical immersion rather than the Wiimote, it needs enough processing power to display many many troops fighting away and lots of debris scattering everywhere.
It'll be fun on Wii, but it won't be immersive. ^_^;;