I use to work for Best Buy (almost 6 years), if there is an ad for this device already, there should be a SKU generated already as well. All it would take is someone, a regular BBY employee, to check the SKU within their inventory system. I forgot what the system is called, but you basically can search by SKU or keyword. It would reveal a ship date/arrival date and amount incoming/on-hand.
All of BBY ads start on Sundays. Based upon the pictures in this thread:
http://forums.androidcentral.com/verizon-galaxy-nexus/135504-verizon-will-call-nexus-prime.html The SKU appears to be: 3904141 It doesn't show up on Bestbuy.com but it may show up within the internal inventory system.
Nonetheless, I find it highly doubtful that this phone will launch at BBY as the "Nexus Prime." Best Buy appears to be misinformed.