Some interesting discussion on whether holding out for a later model hybrid is worth it. The 2010 Prius is expected to get 50 MPG, which is widely considered a major benchmark. Upon reflection I'm not really sure why.
Setting aside the "feel-good factor" (which some people are certainly willing to pay for--witness the success of "fair trade coffee"), if you look at the raw money-saving potential of marginal increases in gas efficiency, it is worth considering that the relationship between miles per gallon and money out of pocket is non-linear. You can do the math yourself if you don't believe me, but here's a graph that assumes one drives 200 miles a week (this is about what I drive these days) and gas costs $2 per gallon (which is what I pay, to one significant figure.)
If the shape of the curve seems non-intuitive, what you need to keep in mind is that to get dollars/week, you have to multiply miles driven by gallons per mile (the reciprocal of miles per gallon) by the price of gas. (Remember dimensional analysis kids?)
So if I were to trade in my trusty old Jeep, which gets 16-17 MPG for pretty much any late model Japanese 4-cylinder car (which generally get about 30 MPG, at least on the highway), I would save $12.67 per week on gas. Not bad! But if I were to get the 2010 Prius, I would save...$17.00 per week. In other words, the difference between the Prius and the old-fashioned 4-banger is only $4 a week, or a little over $200 per year.
Of course, if the gas price jumps back to this year's high of ~$4 per gallon, you can multiply all of those figures by 2. In my opinion, still not worth it for the hybrid, but almost certainly worth looking into losing the Jeep for a 30 MPG used car of some sort.
Similarly, aggregate demand for fossil fuels could be reduced much, much more by getting all the vehicles that currently get less than 20 MPG (mostly trucks, SUVs, sports cars, and larger sedans) up to 25 or 30 than by getting the 2-door coupe already at 30 up to 50.