The PHEV Prius prototypes from Toyota were revealed on July 25, 2007. In the video shown, at 1:07, you will see a Toyota E-COM, which was an EV designed for car-sharing/city. Toyota made about ~50 of them or so. They had a few of the E-COMs hanging around until 2006, when they ended the E-COM program. Here are a few pics of the E-COM (1, 2)
So, if the E-COM program ended in 2006 and the PHEV Prius is in a video with it beside it on the road, that must mean that the Prius PHEV prototypes must've been at LEAST 6 months old when they were displayed on July.
I know that there was already speculation beforehand that Toyota was developing PHEV prototypes, but this confirms it more.
Here is the website saying when the car program ended.
I'm not sure that just because the E-COMs are in a video with a PHEV Prius proves anything. When Toyota ended the E-COM program, they didn't destroy the cars, did they? If they mothballed them, then Joseph is onto something. Whatever the case, I've asked Toyota if they'd like to comment on this. For me, though, the excitement is in the future of the plug-in Prius program more than in its history.
[Source: EV World, Toyota, tipster Joseph]