I been having this problem with WiiMC since mid summer. After about 10-15 seconds, a stack dump screen with show up and freeze the Wii completely, forcing me to hold down the power button to be able to reset my Wii. I tried formatting my SD card and getting a clean card, but it still happens. I figured it might have something to do with my USB drive, but it happens whether or not it or anything else is plugged in to the machine. Other media players, such as Mplayer or Geexbox (ugh) works however without any apparent issues.
Here’s the dump code I’m getting, note that the Wii is not able to reset from here. I’m on the latest Wii update with no modifications to the system, just vanilla homebrew channel and WiiMC on the card: