Sony may indeed be planning to release a new series of slimmer, more efficient PlayStation 3 consoles, if a filing with the FCC is anything to go by. The filing implies that the 4000 series PS3 will have a new case and that the wi-fi will not be upgraded to the N standard.
The above is a picture submitted for the new 4000 series. It actually seems a little bigger than the 3000 consoles, the current slim series, although that might be down to scale and the way the image has been posted. With Gamescom only about a month away, any official comment on this will be held until then, but it wouldn't surprise anybody to see a price cut and a new, cheaper console. There have been rumours for months and no doubt Sony are interested in recapturing a few of those early Xbox-adopters before the reveal and release of the PlayStation 4.
For comparisons to other FCC filings and discussion on what this could all mean, check out Neogaf.