In order for Toyota to build the Prius in the US, they would need to build smaller, lighter parts according to Toyota Vice Chairman Fujio Cho.
Source:Wall Street Journal
"Given the popularity the car has proven and given the way American consumers have snapped up the car, I have been urging the company, almost as a matter of strategy, to produce the Prius in the U.S.," Mr. Cho said.
Toyota is hoping to build 70% to 75% of the three million cars and trucks they expect to sell in the US domestically. If demand continues to increase as it has, Cho expects Toyota to build an eighth plant here. They currently have five, with two more being built.
The Prius is currently being built in Japan because the parts are being built in Japan. They are just too bulky to be shipped into the US. If the batteries, inverters and electric motors shrink and become lighter, production would naturally move to the US.