Well, the speculation can end. The folks over at Gamerscoreblog announced today
that the free game for the three+ weeks of Live headaches will be Undertow
. The game will be free for a limited time, from Wednesday 1/23 at 5:00 a.m. EST until Monday 1/28 at 2:59 a.m. EST. The game normally costs 800 Microsoft points and is one of the highest rated and visually good looking XBLA titles. I'm sure this will be up for some debate, so feel free to leave your shout out in the comments section:
EDIT: It was noted by John Porcaro in the comments section of the Gamerscoreblog post that if you already have Undertow to call support (1-800-4-MYXBOX in North America), as they will take care of you.