According to an ex-SCEA R&D director's LinkedIn profile, the next generation PlayStation has been in development since August 2010. His profile has a listing citing his work on the "Next Generation PlayStation" from August 2010 to April 2012. His work on the PS Vita is listed separately which implies that the "Next Generation PlayStation" he is referring to is the PS4.
"In 1998 I started as the first engineer in the Playstation US R&D," reads Vass' profile. "I worked on graphics ( COLLADA, PSGL ) and a lot of network related technologies ( advertising, telemetry, PVRs ) for the Playstation2-x, PSP, Vita and Next Generation platforms. Platform security was occupying most of my time at the last years... "
SCEA boss Jack Tretton also recently let loose some information on the next generation system when he said that "we've never been first, we've never been cheapest, it's about being the best. If you can build a better machine and it's going to come out a little bit later that's better than rushing something to market that's going to run out of gas for the long term."
The LinkedIn profile has since been changed to say January 1917 for the start date of the project. He hasn't released any information that should get him in trouble, however, if he goes to work for a company that values secrecy, say Apple, then this could possibly come back to haunt him. It seems as though the PS4 is still a little while off.