here is the big secret:
Actual sustainability = frugality over the long term.
If it costs more over its lifetime, it is NOT sustainable. So, when gas is at $2.50 a gallon, the Prius is NOT always sustainable.
Proof: The cost of any item, in the end, is the energy required to make it. All the metal, plastic, glass, etc.. needed to make a Prius is just setting in the ground, free for the taking. The real cost of making it into a Prius is the cost of the energy required to dig it up, ship it to the foundry, refine it, ship it to the parts factory, form it, ship it to the car factory, assemble it, ship it to the lot, and sell it to you. All of the equipment and people involved in those processes are, again, the cost of the energy required to make and operate.
Just about ALL of that energy is fossil fuels. So how does it make sense to spend $22,000 (http://www.toyota.com/prius-hybrid/trims-prices.html) worth of fossil fuel to get 51/45mpg, when you could spend $12,000 (http://www.toyota.com/yaris/trims-prices.html) worth of fossil fuel to get 29/35mpg? You are saving about 20mpg, or 12.5 cents per mile, for an expenditure of $10,000. You would have to drive 80,000 miles to make that worth doing. Given a 5 year average vehicle life, you would have to drive 16,000 miles per year or an average of 45 miles per day. So if your commute is less than ½ hour each way, a Prius actually HURTs our Mother more than it helps.
Of course, the numbers REALLY make a jump when you consider a USED car vs a NEW Prius. A 25mpg 2001 Camry for $10,000 with an average useful life of 20 years turns out to be a much more sustainable choice than a new Prius for that same 30 min commute.
The REAL green people drive old cars and support their local mechanic until the repair bill exceeds the savings.