The best way to determine when any tax break is available is to consider which option of implementation of such provides the greater benefit to the government. You'll always be right. For instance, the tax break is often listed as a tax credit. It is not. It is a deduction. Oh well, we should take what we get and be thankful.... At least I think we should.
As a CPA who has been on a Prius waiting list since 11/03, I have checked this out. If you took delivery in 2003 of a new (not used) Prius you enter $2,000 on line 33, just to the left of the total column, on the front page of your 1040. And in the blank space to the left of it write the words "clean fuel". It then gets totaled with any of your other adjustments and the total is entered in the far right column and then it is subtracted from your income. If you took delivery in 2004 the amount is only $1,500. If you live in a state that taxes based upon the federal AGI you may get a state tax break also.