The Playstation Blog has a lengthy video interview with Shuhei Yoshida, the head of Sony Worldwide Studios about the development cycle for the upcoming PlaStation NGP. In it, he reveals that Worldwide Studios first began working on the device back in 2008:

Since the beginning of 2008, WWS have been officially involved in the development of NGP by SCEI, and not just with developing games, but in the development of the various aspects of the platform itself too, such as hardware performance and component choices, form factor, usability tests, connectivity, user interface and features, target audience, business models and software product portfolio. In short, everything.

[source ngportable.com, via uk.playstation.com]