Two anonymous retail spokespeople in the UK have reported that PlayStation 3 supplies from Sony have dried up, possibly in anticipation of this month's GamesCom and a future announcement therein. "Sourcing PS3 stock has been a challenge for the last two to three weeks," one source told MCV. Evidently, the retailers have been told that further shipments would not be coming until late August or early September.
We can only speculate about the reason -- you may expect the next shipment of PS3 systems to make it to retail to be Slim, or a cheaper model; however, given recent history, we're going to conjecture that Sony is planning to release a new model with a higher price, and doesn't want to waste any more time building the "cheap" systems.