Here is an idea, someone who doesnt have an XBL account, get a packet sniffer(ethereal is a good one) sniff everything while u sign up for an account. try to make sure nothing else is running that will be captured. It will likely be a shitload of information to sort through. Hopefully if its not encrypted, it should be easy to pick out it sending a serial number of some sort.
Then also sniff when you log into xbl, see what it does.
I'd try it, but i've been on live for a bit, wouldnt work out.
If not, it would be interesting to see what goes back and forth during initial setup.