Just saw this on Hack-a-Day. Using a Teensy board, someone made an ADB to USB converter. Since the Teensy is only $20-ish, if you're at all handy it's a lot cheaper than rare Griffin adapters that occasionally show up on ebay.
Don't know if I'm tempted enough to try it, but I do miss using my Extended Keyboard II.
The board is small enough that you could embed it inside the keyboard case.