This year's so called third generation Prius was originally expected
to deliver with lithium ion batteries. They upgraded every subsystem
except the batteries. I think lithium ion batteries are ready to go
but some safety issues are lingering. I'm guessing another year of
safey testing and tweaking and we will be there. Lithium ion
batteries should boost the Prius (and the Insight) into the 70+ MPG
range.