I received a call from my Honda salesman yesterday to tell me about the tax credit. That's great service! He called me within hours of getting the information. The credit was better that he'd originally expected, but was on target with other predictions. The difference between the Honda and the Prius is their emissions score balanced with their estimated MPG. The rating for the HCH2: http://www.epa.gov/emissweb/E-HONDA-CivicHybrid-06.htm
The rating for the Prius:http://www.epa.gov/emissweb/E-TOYOTA-Prius-06.htm
I was surprised that the Prius received a lower score outside of CA & NY. The Feds lump them all together, though. It's got to be weighed as emissions per estimated mile driven.