In the wake of extremely positive results after their announcement of the PlayStation 4 at E3, Sony has revised their internal sales estimates for the console device. They also admit that it's very possible that demand will outstrip supply for the units.
Global PlayStation boss Andrew House stated that the company is shifting its internal projections for the product after favorable reactions to the company's presentation Monday".
Retailers, such as Amazon and GameStop, are concerned that they will not be able to fulfill the day one pre-orders and are now warning customers that they cannot guarantee shipment of the units on the promised release date.
Expect a few overpriced sales on Ebay. On a side note, the Xbox One doesn't seem to be having this issue as in not currently on top of the best sellers list at many retailers.