Are you a retail manager? AT&T employs 276,000 people and not all would be affect by the release of some popular handset.
Not that I can't imagine that this rumor is a total crock. That said, this is the timeframe where Apple has previous shipped iPhones. Retail employees are typically the last to find out about such matters.
I've commented elsewhere that this rumor says nothing about which employees were notified. It could be network engineers, purchasing agents, or inventory control specialists.
Basically, someone from Apple Channel Sales could have said, "Hey, we're sending you 250,000 iPhones in a couple of weeks. Make room in your warehouses. There are X units per pallet, each pallet weighs Y pounds. Oh, yeah, please transfer $120 million to our bank. Call us when you need more. We expect to hear back from you in a few weeks."