General Motors is forging ahead with lithium ion battery development for both the plug-in hybrid Saturn Vue
and the Chevrolet Volt. Meanwhile Toyota's executive vice president for R&D, Kazuo Okamoto is insisting (subs req'd) that they are still too hazardous for automotive use.

Toyota is still working on getting better performance out of the nickel metal hydride batteries they use in their current hybrids. Plug-in hybrids do more cycling of the battery as they charge and discharge than current hybrids. That puts more strain on the battery and raises temperatures. For now, Toyota is testing plug-in Priuses with higher capacity NiMH batteries and won't give a timeline for moving to lithium. Okamoto also listed cost as an additional factor limiting lithium.

[Source: Automotive News - Sub. Req'd]