Best Buy offers great specials and sale prices sometimes offered in "Bundles" then refuses to give a raincheck because one of the items is not available. This has happened to me twice.
Most recently (July 2011) Samsung 46" HDTV on sale with free Blue Ray player and 2 free 3-D glasses. Got to the store and the tv was not in stock. Asked for a raincheck but was told they couldn't raincheck the tv because it was a bundle. The had the blue ray player and glasses in stock but not the advertised tv.
This happened to me once before when trying to purchase a camera. If the free items are things that Best Buy regularly carries, they should be able to offer a raincheck to show they are not just baiting you to the store.