The regular plain hybrid 2012 Toyota Prius has a rating of 5 stars from the National Highway Traffic Safety Administration (NHTSA). The Prius plug-in is almost the same vehicle, but it has a different battery pack which can be recharged by plugging it in a power outlet. Well, that different battery pack adds 120 pounds to the total weight of the Prius, and apparently that's enough to change slightly the dynamic of crash tests and earn the plug-in Prius a 4-star rating from the NHTSA.
According to Toyota, the extra weight changed how the car was damaged by the test barrier used by the NHTSA, but "Importantly, all of the occupant seating positions of the Plug-in model receive the same safety ratings as the regular Prius model". The plug-in Prius still received 5 stars in side impacts, and it seems reasonable to expect that in real-world situation, its safety would be very very close to the regular Prius, which has a good safety track record.
Here are the videos, for you fans of broken glass, smashed metal, and crash test dummies: