Dont know if they changed the wording in the past few days but currently the "guarantee" reads:
Just like some pre-owned car dealers put their vehicles through a rigorous 125-point check, we put our pre-owned Xbox 360, PS3, Wii, PSP and DS games through a rigorous check, too, but we've narrowed it down to four points. Game works: Check! Game has a box: Check! Game has an instruction manual: Check! Game is packed with love, the secret ingredient: Check!
So they'll give you a box, but not necessarily a box that corresponds to your game's system. They'll also give you an instruction manual, but no mention of the cover art.
UPDATE: Upon a second look at Cheapy's original link, it seems that they substituted "a box" for the previously promised "cover art". Shady as hell. If you do go to complain, make sure you print out the "guarantee" from Cheapy's link (and take the link too to show it's still on their site). Otherwise, they'll claim ignorance and just go by the revised version.
I'm usually one who think GS hater here on CAG are way over the top, but this right here is a total fraud (if not in the legal sense, definitely in the common sense).
For shame GameStop. You've pissed away what little goodwill you had gained: in less than 3 months even.