Here's a quick fix... for what I'd consider a design flaw.

---- Warning: Do the following at your own risk

Opening my Wii, I noticed that the thermal pad between the heatsink and the GPU+CPU was between 1.5 to 2mm thick.  That's too thick.

Solution (which will be apparent to those who have disassembled their wii to the heatsink level

Materials used:
Thermal compound (lots of good ones available at computer hardware stores.  Choose one you like)

Tools used:
Hacksaw
Metal file
Plastic card

1) Remove heatsink.
2) Remove thermal pad/paste from GPU, CPU and heatsink - SAVE SOME.
3) Remove circular carbon rings + black double sided tape from the 4 legs of the heatsink.
4) Saw off heatsink legs.  Remove burrs & smooth the ends of the heatsink where the legs were.
5) Spread a thin layer of new thermal compound over the GPU and CPU with the plastic card (or something with a hard straight edge that's wider than the chip)
6) Replace heatsink - it can now sit lower without the legs (hence, you no longer need a 1.5 ro 2mm thick thermal pad).
7) Wiggle/Twist the heatsink slightly (about 5 degrees)... to push out any air pockets and ensure a good contact between the heatsink and the thermal compound
8) Screw the 4 screws that hold the heatsink in place back
9) Spread some of the old thermal pad/paste that you saved on the WIFI chip if needed (my WIFI chip was not covered very well... doing this increased the contact of my WIFI chip to thermal pad/paste to the metal RF shielding that covers the motherboard.

My Wii, which used to run hot, is now cool to the touch... even after hours of play