This is an old post, preserved for reference.
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I started the code for the ZX81 USB keyboard, with the USBKeyboardDemo1 sample in the Arduino Version of the V-USB library. As stated in the original post, either change the config.h file in the library to set USB_CFG_DMINUS_BIT to 3 rather than 4 or wire up the circuit with 2 and 4.
Download the source code here: ZX81_Keyboard.ino
ZX81 USB Keyboard by Dave Curran is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 3.0 Unported License.
This was a very primitive version of the USB keyboards I produced later on. The code has been left here for reference, but suffered due to the way the keyboard library worked. I later rewrote the whole thing to have more control over the ways the keys were mapped. That version has not been open sourced.