...is actually kind of hard!
Well, you may or may not know that Nintendo's Wii Remote uses standard Bluetooth to communicate with the console. Enterprising open-source developers have exploited this fact to write drivers for the device, and the most mature on the Mac is Darwiin Remote.
This program will let you communicate with the remote and map the buttons to different keys on the keyboard. It also displays a nifty graph of the motion sensor output, and while nothing is currently done with this, I bet someone will come up with something eventually.
Anyway, being the nerd that I am, I thought it might be cool to try it out, and SketchFighter seemed a natural candidate. The verdict? It works! But...it's a little hard to play. I feel like I felt when I first started playing SketchFighter, and I'd probably need to relearn everything. Nevertheless, the controller buttons are more responsive than keyboard keys, and allow for more precise aiming. If only I could get the flying down.