Once again production is falling behind the demand, leading to long waiting lists for the newest generation Prius. And once again, its parts that are limiting how many Prius can be built at any one time. In this case, it's the battery that's put a cap on the number Toyota can produce each month (Source:Battery bottleneck limits Prius output - Automotive News).

The batteries are made by Panasonic EV Energy Co. And they simply can't 'churn out more' batteries than what they've already planned out with Toyota. At 500,000 cars a year, "The new Prius model is selling quite well," Takahiko Ijichi, Toyota senior managing director said.

I can only imagine what the limitation would have been if Toyota had switched to Lithium-Ion, instead of sticking with the Nickel Metal Hydride battery packs.  They also wouldn't have been able to cut costs on their battery production.

Battery production is set to increase to 1 million batteries a year by the summer of 2010.

BTW, Ijichi also mentioned the Prius is 'probably in the midlevel of profit' across their line-up. That should be of interest to those who still claim Toyota loses money on the Prius.