The offer is available between November 21 and December 21, 2007. Be advised though, that getting the game for free might cost you more than you expect and Xbox.com makes no secret of it.
The official announcement reads the following:
“Current Paid Xbox LIVE Members on the original Xbox console (not Xbox 360), here’s how to get your Halo 3 game:
1.Buy an Xbox 360 console between November 21 and December 21, 2007.
2.Convert your current paid Xbox LIVE account to an Xbox LIVE Gold subscription.
3.Register here between November 21 and December 21, 2007 with promotional code XBX7777, with your new console serial number, and a valid mailing address.
4.Look for your copy of Halo 3 in your mailbox!”
If the new Xbox360 owners go with the offer, they should get the game by January 18, 2008.