The move follows AT&T's plans to sell one phone per customer at the full price for existing customers of its service. In Apple's case however, there is no limit on the number of phones a buyer can purchase, and no need to have an existing AT&T contract.
Activation can completed by the customer at home through iTunes.
While the phones are still technically locked to AT&T service, it is possible (but not supported) to unlock the phones for use with other mobile service providers. In the US, that is largely limited to T-Moble, although that GSM provider does not support the same 3G network the iPhone 3G needs to connect at faster than EDGE speeds.
Apple's change in sales policy comes as the company is working to sell off remaining inventory to prepare for the upcoming launch of the new 2009 iPhone, expected to be released around the middle of June, possibly at the company's similarly-timed Worldwide Developer Conference.