Make sure you're well-known before attempting this, and in a good way. I never had success asking at Gamestop, but that's because I leave empty-handed about 4/5 of the time I go in there. =P If you're perceived negatively, they'll just say no; if you're not known at all, you'll just hear the company line ("Sorry, we're supposed to return them to the company"-often spoken by the same person who intends to take it home after close)
You might try other places for these displays. Big-box stores will usually say the same company line (I get denied at Toys R Us every time, for example) but some of your smaller/non-franchise video stores can be a hit. My family scored some movie displays from one such store when we needed some for a school event. Maybe if you say you're on the prom committee and you want to run a video game theme? *shrugs*
If all else fails, remember: these are just big pieces of cardboard. There's nothing inherently special to them except some value as conversation piece and as a limited, somewhat impractical, means of storage. If it sounds like I'm disillusioned, I am: I tried to get the New Super Mario Bros. display last year and got denied at every single place I tried. After that experience, it's no longer worth it to me.