I found some 2port "Gamecube controllers to USB" adapters at a semi-local discount store for $5.00.
On a whim I purchased one, and discovered pleasantly that it really works well.
I only have one controller in the house now, but it was very much plug-in-play using rpix86. It looks like a great solution for multiplayer gaming on the Pi.
They look like this.. http://ecx.images-amazon.com/images/I/4 ... SY300_.jpg
I'll bet these would be an excellent way to interface to the existing gamecube ports for the next person who puts a Raspberry Pi in a Gamecube case.