GameSpot has put together a pretty thorough roundup of the many retailers, both on and offline, that have started to sell the PS3 Slim before its vague "early September" release date. According to GameSpot, Amazon and GameStop's online stores have the PS3 listed as in-stock and ready to ship. Additionally, calls made to San Francisco GameStop and Best Buy retail stores have confirmed that some brick-and-mortar locations are selling the new system too. The questioned GameStop and Best Buy retail locations have stated that they only received roughly a dozen systems per store.
When the system was first announced at GamesCom, Sony told the general public that the system would be available in the US during the first week of September, while GameSpot was told "September 1 isn't an official street date." Whether it's overeager retail workers pushing the system out early or Sony being lax about setting a firm release date, one thing is for sure; the customer wins.