The company said yesterday that it will include a $25 coupon with the purchase of Halo: Reach (be it the standard, limited or legendary edition). That coupon will be handed out 9/14 through 9/18 and it can be used the next time you buy a video game at Kmart -- though, it's not clear whether all games and platforms apply.
Additionally, if you buy another (qualifying) game at the same time as Halo: Reach, it will be discounted by $25. This specific offering seems to be limited to Xbox 360 titles, but they're mostly big releases. Some eligible games include MW2, Singularity, R.U.S.E., Just Cause 2, H.A.W.X. 2, and Metro 2033. There's a complete list here.