FCC documents filed by Sony point to a new slim model PS3. The folks at pocketnews note that the new documents do match the currently available PS3 (the label position has changed), which means we could have another slim model coming up. The new model number, CECH-4001x also seems to have moved up from the currently available 3000 series.
According to Pocket-Lint, the documents mention that the 'x' in the model will be replaced by an alphabet denoting the different hard disk sizes to be made available. Sony also asks for a confidentiality grace period of 180 days to keep things under wraps. They say that the public disclosure of these documents "might be harmful to our company and would give competitor an unfair advantage in the market." So much for that!
The FCC docs do not give us much in the way of details beyond this. Some point to the last system update, which changed "eject disc" to "remove disc." This could point to a cheaper top-loading unit, not unlike the one Sony released for a PS2 update.
We're betting this all points to a new PS3 set for holiday release. I'm guessing we'll see its official reveal at gamescom next month.