Andy Brown wrote an article detailing how to hack the HP Z800 Xeon motherboard into a standard case:
I’m not one to give up in the face of a technical challenge and besides I’d just forked out a hundred notes on the board so the rest of this article will go through all the steps in detail that you would have to do in order to get one of these beasts up and running yourself. There’s no cheaper way to get 12 cores of Xeon power under your desk.