Apple Desktop Bus (ADB) to USB Adapter
The Apple Desktop Bus is well documented on Apple's website (http://developer.apple.com/legacy/mac/library/technotes/hw/hw_01.html).  It's a single wire communications bus - the other wires are for power and ground, and the computer polls the devices continuously for data after an initial setup period.

It would be relatively straightforward to do this with a variety of microcontrollers, and most USB capable microcontroller manufacturers have at least one application note on how to make a keyboard HID device (ie, USB keyboard).

Microchip's app note TB056 has a PS2 to USB keyboard adaptor.  Replace all the PS2 code and circuit interface with ADB code and interface, and you should be able to accomplish exactly what you need without a great deal of effort.

http://www.microchip.com/stellent/idcplg?IdcService=SS_GET_PAGE&amp;nodeId=1824&amp;appnote=en011984 is the current page for that app note, but if the link stops working you can search for TB056 and find it.