The Mac mini and Apple TV would be good products for Wal-Mart and Target, along with the iPods they're already selling.
Costco occasionally does buy Macs. A friend of mine missed my daughter's wedding last April because he was trying to find a Costco in Sacramento that still had a 20" iMac G5 in stock. The 50 that had been offered for sale at the Stockton Costco had been sold in under six hours. He found similar stories when he called around to other Costcos within a 75 mile radius.
He bought iMac at the South Sacramento Costco store and missed the wedding because the manager would only hold it for two hours. He did get to the reception... and brought the boxed iMac in because he didn't want to leave it in plain sight in his truck in the parking lot. ;^)>
By the way, he was switching from a PC. He now has two Macs... he bought a Mac Mini as well.
I have seen a stack of Mac Minis at the Stockton Costco.