I have actually considered the same thing, and it seems within grasp of a homebrew project.

Isn't USB, ADB, AT and PS/2 all 5 volt?

The only issue seems to be signal conversion for the USB pinouts, of which there is likely a crapload of documentation out there sufficient to create a working IC.

This is something people would probably pay for. PC users still love their Model M keyboards as much as we love our Apple Extendeds. There are a number of PS/2 to USB converters.