According to Sony, the demand for the PS3 is tremendously high and they expect all retailers to have a low amount of inventory in the coming months.
"Yes. The strong demand carrying through the holidays was evident and early in the month we knew there was going to be limited supply of the PS3"
These "supply issues" could get worse with the up and coming release of games like Heavy Rain, God of War 3, and ModNation Racers.