What is a Plug in PHEV Prius?
Too my Knowledge...
All current (stock) Prius do not plug in, so they are just HEV. (Hybrid Electric Vehicles)
There are aftermarket kits to add more batteries and a plug in charger control, to make current Prius able to plug in. They tend to change handling and be non-seamless in operation.
Toyota is working on a factory Plug in Hybrid Electric Vehicle, due in 2012. Ideally the handling will be less effected and the control will be seamless