Supply & Demand
A few months ago, Toyota was quite afraid of the new low-cost Honda Insight hybrid. It even went as far as to make a $21,000 version of the 2010 Prius (aka Prius I) to try to compete with Honda on price. Well, seems like Toyota isn't too scared anymore, because it has announced that the Prius I will only be available in a limited way to some fleet buyers (in practice, this means you can't get one anymore). It is also making changes to the base price of the other Prius 3rd generation models (Prius II, III, IV, and V).
The fact that the 2010 Prius is selling so well, especially in Japan where sales exploded recently, probably was a main factor in Toyota's decision to increase the price of all the different Prius models by $400. "Base prices for the Prius will range from $22,400 for the Prius II to $27,670 for the Prius V. "
After all, if you're selling everything you can make (demand is higher than supply), it makes sense to increase price a bit and, hopefully, redirect some of that extra money into increasing the Prius supply.