Just got my Wii and while setting it up found myself stuck in 480p resolution when my TV only supports 480i. (Anyone that has ever done this will know that you're not going to see anything too clearly when this happens.)
Switch back to the original cables that came with your Wii, turn it on, turn it off, and reconnect your component cables. This will cause the Wii to default back to 480i, even when you reconnect your component cables.
I'm sure this has been addressed somewhere on the forums, but I'm a newbie and couldn't seem to easily find it after a few searches.