Gamestop has been doing this since midway last year. They have done it on such movies as Troy, I-Robot, Anchorman, and recently Friday Night Lights, Shark Talk, and Sky Captain. They do the 3 for free deal on any movie they can preorder, which is a minimal list. About 3-4 new titles per month.
I went to my local GameStop and they only have the trade-in deal for Ladder 49 and Sponge Bob. The guy said I can pre-order Incredibles but not trade-in 3 used DVDs for it.