If you were going to pick up an Xbox 360 for Halo 3, surely you would have done it by now?
I can’t imagine there would be too many people left who would buy a new 360 just to get a free Halo game (most of you would already own a 360 by now, surely), which is probably why Microsoft didn’t go with the official bundle option. But as David over at Kotaku has informed us, Microsoft is giving away a copy of the Halo RTS (which is all kinds of enjoyable, btw) with every purchase of a 360 console between Feb 26 and March 26 (or until stocks run out).