I have little to Joe's excellent point, but one observation.

There are many "Moore's Laws" in technology, which you can learn about if you read Kurzweil's book. :-) One of the observations is that batteries improve over time.

So 8 years from now in 2012 I won't need to spend $11,000 for a replacement of the crappy old battery pack that was the best Toyota could find back in 2004. Before that time I'll have recycled the current battery and put in something with daily EV range that also increases the Prius' mileage a bit. The Prius is designed so upgraded batteries can be dropped in. As Calcars.org has shown, the Prius has the extra space and a distributed battery management architecture so the engine does not need to be hacked for the car to take advantage of a superior battery. Obviously, the scrap value of the existing battery pack will cut seriously in to the cost of its replacement.

The kind of euro carbon credits smart people have been buying ( http://www.ieta.org/ieta/www/pages/getfile.php?docID=1182 ) are a good deal due to adequate verification.
Chicago carbon credits are underpriced due to lack of buyers persuaded by its verification methods- it eventually should cost more
but for now- by all means, enjoy the bargain.

The capital expense of any new car is partly a real large energy expense for the materials and manufacturing and marketing that went in to producing it.
Rather than a new Camrey, a safe clean and unpopular used car may be a better deal for the environment, if you invest the difference in verified carbon credits.
If you live in an area where it is safe to drive, an electric bike is much better choice.
However the existing style of cars, especially if replicated in China, will doom the planet.
We can switch to new tech which is at hand, we lack only the political will to use it.
I believe the Prius represents a number of seriously important incremental steps towards the life saving tech we need.
PHEV's are a lot closer and get people thinking about sustainability.
Building a market and a supply chain for PHEV's and EV's will take time but will soon beat buying a beater and some carbon credits.

I want Toyota out there inspiring battery manufacturers to help them make better hybrids.
I want them proving out the electric regenerative power breaks and electric power steering and electric air conditioning that will make EV's succeed in the US market.
I'm not so happy about the deals that apparently keep the best batteries out of the hands of other car manufacturers, however.
So buying a Prius sends the best message I can to the car companies right now. There is money in technologies that don't suck.
If the Prius isn't the greenest, I'm eager for the auto-marketing departments to help Americans find one that is better. I don't mind the fact that no "luxury car" is as quiet and smooth as an ordinary Prius in stealth mode. Building a demand for more clean stealth mode driving draws us closer to the right economic solution.