After looking around the web the last few days for cheap deals on PS3s i have noticed that a few retailers have reduced the price on their PS3s to clear old stock, but the old stock appears to be 40 and 80gb PS3s. I have called around a few stores and have heard from at least two that the reason was to clear the old stock for a new PS3 SKU(stock-keeping unit) one told me that a new PS3 SKU will be out very soon, i then asked if this was to do with the PS3 slim in which he told me "well if i was you i would keep an eye on GDC Europe 2009 for more details" this is what he was told by his employer.
So the evidence appears to point at the PS3 Slim being announced at GDC Europe 2009 with a release very soon after maybe around september at a price of around £249 with all old versions being sold at a lower price to clear.
Otherwise expect an annoucement of the PS3 Slim to launch late Sep/October at the Tokyo Game Show (24 Sep)
Either way it looks very likly we will see the slim by the end of October at the latest.