Agree; these very high mileage vehicles only make sense at very high gas prices and for high mileage drivers. Where I live the cab drivers are now switching wholesale to Prius vehicles. They put on 100,000 miles a year or so, with a lot of stop and go city driving. A diesel hybrid option would pay for itself after a little over a year for them. The current taxis are mostly Crown Victorias, getting an average of 20 mpg on regular gas. Using 5000 gallons per year @ $2= $10,000 in fuel compared to $4000 for a Prius is a complete no-brainer of course.
Personally, my driving (in my own vehicle ) is down to 7000 miles per year. Last year I spent $859 on gas, so a gas Prius will save me about 60-70 gallons per year or $120-$140 over my 4 cylinder Toyota. The payback for me for a Prius ($9000 premium) is 64 years at $2/gallon and only 32 years at $4/gallon.
Diesel hybrids are being developed by all European car makers, since diesel is priced much lower and gas prices were $8/gallon last year. We need to go back to $4 gas in North Amercia for diesel hybrids to make any sense.