Here is a link to a couple write-ups about the credit. From what I see it is for plug-in [i]hybrid[i]vehicles. And of course one must have a minimum sized battery pack. I have seen both 4Kwh and 6Kwh listed as the min.
Hybrid Car Blog - Hybrid cars and plug-in hybrid vehicles news and info: $7,500 plug-in hybrid tax credit passes Senate - Too favorable to Volt?
JJ