The Elgato EyeTV 250 will give you Mac a composite input to hook up the Wii. These are usually used for video capture or TV viewing on your Mac. Nice thing about this is that you can record. I wonder how this would effect gameplay though, there has to be some lag in the conversion process.
If it's a newer Cinema Display it would be expensive to try and run the Wii directly into it. You would have to convert component into DVI-D. Those boxes are $400+. If it's an older display with VGA (not ADC) they you are in luck. The StarTech COMP2VGA will go from composite to VGA. Max resolution is 1280x1024 so if your using an LCD make sure your native resolution is at or below that. I've never used one, but I hear they work pretty well. I might pick one of these up so I can use an old 21" CRT I've got sitting around.
