It could be the ballast, but it could also be that you got a bad bulb or even an incorrect one. The bulbs vary by year although not every year. These bulbs don't have a filament so it's the arc that creates the light. It's worth complaining about (assuming you got it from the dealer).
You can purchase bulbs from a variety of places such as:
http://www.oembulbs.com/ or eBay.
Instructions on how to change the bulb are here:
http://priuschat.com/forums/gen-ii-p...-06-prius.html
I replaced the OE bulbs at 75,000 miles and they are still fine at 115,000. Note: I drive with the lights on all the time.