Consumerism Commentary has an article about the $10,000 hybrids  that Honda is planning to roll out in 2007 and wonders aloud if it’s worth it to switch from a 2004 clunker getting 30 miles per gallon to a sleek 2007 clunker getting 60 or 70 miles per gallon. It depends.
At $2.50 a gallon, a 30 mpg car pays 8.3 cents per mile driven. If you consider that the average car drives approximately 12,000 miles a year, that puts the total gasoline price at $1,000 a year. For a 60 mpg car, the total cost of gasoline is $500, a difference of $500 (not $1,000 as I erroneously wrote here before) So at $2.50/gal, the car would pay for itself (initial cost, not including taxes, titles, etc.) in twenty years. At $5/gal gasoline, that’s in ten years.
So is it worth the change? Probably, but it depends in part on how much you would “lose” if you sold the car in 2007 and bought the $10k hybrid. It also depends on whether the hybrid tax breaks  are still around by the time these cars roll off the factory floor. You can also add in all sorts of other financial calculations like the interest earned on money not spent on gas, etc. but if a $10k hybrid car exist that performed well enough (I’ve never driven or rode in a hybrid car) then I’d switch in a heartbeat.
[Edited because my math skills are weak, must be all those MBA classes I’ve been sitting through.]