The rain checks are a little YMMV. SHC mentioned the stores are supposed to give them but the Kmart I went to a few days ago refused to give me one, after much waiting. I was hoping to get in on the MW2 double xp week but it looks like that's not going to happen.
Yup, same thing happened to me at my Kmart. I was told that these type of sales are only available "while supplies last" and is not eligible for rain checks.