I am new - sorry if this is redundant, but thought it might be of assistance to those 06 buyers after Jan 01. The wait paid off!! http://www.aceee.org/transportation/hybtaxcred.htm#follow
It is redundant, but that ok... most things are in some fashion or another. Kiloran has posted quite abit about the tax bills. Dig around a bit, it is a quite interesting subject.
Be sure to check out what state and private incentives might apply for you as well. There are details and links in the Government_Incentives section of the Toyota_Prius_FAQ.