OK, I signed up 10 times last week with 10 similar Hotmail accounts and account names (brucevilanch1-10) and the same, slightly altered street address (St., Street, etc.) and five different credit cards, each used twice. The first three (brucevilanch1-3, one credit card for two accounts, one for one) went through and I got confirmations from XBoxAchiever, nothing for the other seven; I guess they were figured out or something. So signing up twice at the same time with the same credit card seems to work OK. I think it was the similar names that alerted them.
I don't know if my credit cards have been flagged or anything, but I'm going to sign up again with the one that went through twice, see if that works again, and then sign up 10 more times after that with completely different names and fake addresses and see how many go through.