Frequently stores get their games the night before the street date, actually. Sometimes a bit earlier than that. Sometimes the employees sit around nervously waiting for UPS or FedEx to get there ON the street date :)
USUALLY there's a lead time, but not always. And lots of times, SOME store (around here, usually Target ;) ) will break the street date and put the product out as soon as they have it.
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--Greg
http://www.pobox.com/~gregleg/