people forget the volt has a $7500 tax credit, so net is about $32,000.
as far as I know, prius's offering will be still based on NiMH, and not offer the range, and be priced pretty close to the middle of options on current prius pricing. (starts at 26). not too certain yet if it will end up qualifying for the same tax credit....
at any rate, its still a little prius. They have no similar plans for a camry or corolla which is what the Volt is closer to in size. Even if they did, unless they switch charging tech radically, or goto Li-on, the heavier car will get even less range in battery only mode.
I'm a betting man that GM will be able to improve charge range by at least 8% a year. Add 3 miles to the 2012 volt. and so forth.

