Here is a clipping from there "New Site Features" section:

"Pre-order select products online and pick them up in-store.
(This feature will be available in several days.)"

I saw this a week ago, so it must be coming pretty soon. I think I'm going to order Mario Kart Wii like this.

To see the full update list go here:
http://www.gamestop.com/gs/landing/features/