I first noticed this on Twilight Princess recently...speckles that appear on the screen. I thought they were random but soon I noticed they were in the same spots when playing Twilight Princess. If I returned to the Wii menu after Zelda, they would be there too, most noticeably to the left of the Disc Channel.
I wasn't sure at first if it was the TV or the component cable or the game or the Wii itself, but it may be a Wii problem, as the specks it creates seem specific per game and yet I didn't have these speckles until a few days ago. There are some in Wii Sports now too and I rented Rayman: Raving Rabbids, where there are many noticeable speckles that fall within a series of vertical bands across the center of the screen. The game is still playable, things are still visibile, but if this is a problem that will worsen then I want to do something about it. I don't want to send it in for repairs but maybe have to (and I don't want to lose my saves or Miis or VC games, etc.).
My brother says people on GameFAQs think it's an overheating problem. I have it vertical, as I always have, but recently I moved the Wii lower down the shelf so that my brother could play DDR: Mario Mix without the dance pad's cable being a problem. Lower down here, there's less ventilation and it's surrounded by the satellite receiver, DVD player, and amplifier. Could that be why, if the Wii is actually prone to overheating? The other guess is that the video card is screwy, which will definitely need repairs.

I never had a problem with Nintendo hardware until I got a DS Lite earlier this year with a dead pixel in the middle of it (thankfully the store took it back without a fuss). I hope Nintendo 's hardware track record isn't slipping; my NES still works. >_<