The Prius MPV, or micro-minivan compared to today's standards will be named the Toyota Prius Alpha, debut in March 2011, and a seven-seat configuration will feature lithium-ion batteries. According to Motor Trend, who it looks like is pulling their information from a Japanese auto mag, the lithium-ion batteries will be 1/3 the weight of the nickel-metal pack used in the 3rd generation Prius.
The Alpha's overall length will be stretched by about 12 inches compared to the current Prius hatchback and be widened under an inch. No word in Motor Trend's article on whether the 7-seat configuration will be Japan-only, or if it will make its way to the US. The overall length of the Prius Alpha would be longer than the Mazda 5, so I don't see any reason why Toyota would not bring at least a 6-seat configuration to North America. The 5-seat version of the Prius Alpha will use nickel-metal hydride batteries to save on cost. Motor Trend's source also says that the Alpha will start from 2.5 million yen, which is $27,625 in US dollars.
The all-new Prius Alpha will also use the same 1.8 liter engine used in the 3rd generation Prius, with a combined horsepower of 135 ponies. MPG numbers probably won't rival the Prius hatchback's 50 MPG due to the increased length and weight, but I would expect it to top 40 MPG.