I actually asked about this over on Remod, Demitri works at GameStop and had this response.
If they do things correctly, out of the copies they recieve (probably today or tomorrow), they put the number of reserves in a drawer or a special section of their game drawers. If they don't recieve enough copies to fulfill every single reserve (which is rare), they do not sell any copies until all reserves are fulfilled. If they run out of spare copies, and only have reserve copies to sell, and demand is high enough, they are allowed to sell reserve copies 48-72 hours after the release date (i think it's three days, but i think it might vary from manager to manager, the minimum is 2 days afaik).