Ok, just called a couple of shops, and they did say that there is a 48 hour policy for holding reserves. However, it sounds like they also do a courtesy call a few days after the 48 hour period (approx a week or so after release) to allow folks who have preordered the chance to pick them up. After that, they may be up for grabs.
Sounds like I'll be calling a few Gamestops next week after I get back from my trip...
I'm hoping I'm the only one asking! I wouldn't mind getting another copy (if it works out that way for more than one store) in case there's someone else out there like me who is regretting not preordering in time.