I know ive got my reciept in my bag with my still sealed Halo Xbox which was purchased at launch. I can check at home if you'd like when i swing by tomorrow. Hell I'll just check considering that I dont think i'll see a response before then.
::EDIT::
I looked at my online account transactions and remember that I paid cash for mine from one of my other accounts (and not my primary account). However I do see 2 transactions at the mall on 4/15 that I did put on my card. So MAYBE 4/15. I'll double check the reciept to be sure though tomorrow.