From the Toyota Plug-in Prius website:
Q: The Prius Plug-in has no spare tire. What if I get a flat?
A: Like all other new Toyota vehicles, the Plug-in comes standard with Toyota Care , which offers complimentary maintenance and roadside service for two years or 25,000 miles. In addition, the Prius Plug-in will come with an extra year of roadside assistance, for a total of three years from date of purchase. In the event of an emergency where roadside assistance cannot be contacted, a Tire Repair System is standard equipment.
Best information I can find on the "Tire Repair System" comes from the Toyota 4-Runner forums. Article from 2002, so not sure this is what Toyota has in mind.