I've been looking in to this a lot. It looks like this is the current
estimate of the tax credits:

http://www.aceee.org/transportation/hybtaxcred.htm

The Civic CNG comes out on top with a credit of $3500. Unfortunately,
with natural gas prices going the way they are, I think it would be
really expensive to operate one of those things. The Prius is next up
with a $3150 credit. The Accord only rates a credit of $650.

--Bill