Very simple, and sneaky!
1) Grab a paper and pencil.
2) Go to GameStop/EBGames.
3) Look for Call of Duty 4 "Game Of the Year" Edition Case (should be on shelf and opened already).
4) Open it up, and look on the inside cover where the manuel is.
5) There should be a paper showing the code.
6) Write it down, and leave the store.
7) Enter the code on your PS3, and download it.
8) Enjoy the freebie!