Found at CDFreaks:
http://www.cdfreaks.com/news2.php3?ID=5199
Read more:Thats Right folks! We have done it. We could tonight successfully change the XBOX Serial Number and MAC Address to something we wanted and the XBOX LIVE Network Dash picked the changed Serial number and MAC Address up!
We Then successfuly signed on to XBOX LIVE with the NEW serial number and MAC address with the MOD Chip removed.. the XBOX LIVE System was 100% happy with the NEW Serial Number! While we were at it, With our New Serial and MAC we played a bit of Online UNREAL Championship (And kicked some ass!) This is a GREAT step forward to help understand why a 100% stock and a un-modified XBOX with all modifications removed can NOT access XBOX LIVE.
Please beware that this is for informational purposes ONLY you MUST only use XBOX LIVE with an un modified XBOX as stated in the XBOX LIVE agreements
http://www.cdfreaks.com/news2.php3?ID=5199