Someone named bunnie has apparently already taken Xbox hacking a step further than anyone else so far. He removed the box's surface-mounted BIOS chip and installed a socket so that he could replace it with a standard PC BIOS chip.
Then, he downloaded the contents of the Xbox's BIOS chip and posted them on his web site, along with pictures and instructions on how he did it.
Within 12 hours he got a call from Microsoft (mp3, edited for content), and the BIOS file is now removed. However, his page also points out that while the Sony vs. Connectix case set a precedent that allows for reverse engineering, it does not allow free distribution. So if you want to follow his instructions and have your very own Xbox ROM BIOS file, you can, you just can't get a copy of his.