The OU haven't released a protocol document for the board, but the squeak source file is available on the OU website, so (while not trivial) it's possible to reverse engineer it to work out how to talk to it over USB.
We've actually been doing this to get the Sense board talking to Greenfoot, and now have a working implementation in Java in the form of a Greenfoot scenario.
It's not 100% stable at the moment (I blame the horrendously buggy RXTX libraries) but if you select the correct COM port in the SenseWorld class, it should fire up. Briefly tested on Windows and Linux, though not on the Pi yet (RXTX fiddling may well be needed as demonstrated here
In the process of doing so we also produced a protocol document which describes the protocol in full - it should be useful to anyone wanting to implement another library with the sense board. Once the protocol is known it's really not that hard to implement - pretty simple stuff just sending and receiving groups of bytes over a serial port (over USB.) You can grab it here
, but please bear in mind it's not an official OU document, just what we've worked out from reverse engineering the Squeak code (so may contain inaccuracies accordingly.) If anyone does notice anything that's wrong, or has any questions, let me know and I'll try to help!