Now, I don't know if this has been brought up before, but I haven't seen it, so here:
http://www.gamestop.com/Catalog/ProductDetails.aspx?product_id=72956
Apparently the limited edition will include, besides the game disc and instruction manual, a 24-page artbook featuring concept art made at different stages in the game's development, a code to unlock a unique and special version of the All-Seeing Eye in-game, and another code to add an exclusive character skin for multiplayer called "Secret Agent".
Pretty cool, but since I live in EU I couldn't get this even if I wanted to, so meh.