Pretty cool trick; seems like your PAL Wii can play GC imports out of the box. No mods, no Freeloader, nothing. Originally found here.
Here's how:
Put an original PAL GC game in the Wii.
From the Disc channel, hover your cursor over the "Start" button.
Now, here's the tricky bit:
Press the Eject button on the Wii, and then press the A button on the Wii Remote to select "Start".
The "Start" button will remain unghosted for a moment after you hit the Eject button. Try to select "Start at the last possible moment.
As fast as you can, whip out the original PAL GC disc and swap in your NTSC GC disc.
You've only got a split second here. Get the NTSC disc in before the disc slot lights up.
BINGO!
If you've been quick enough, the import GC game boots up. Once you get your timing down, this isn't hard to do.
I'm assuming this works for American consoles and Japanese games, too. Wii software apparently doesn't work though.