Interesting point you bring up. I was doing some math... if gas around here goes past $3.50/gal, I will be able to buy a Civic hybrid to commute in, and I will break even over driving my Jeep (10-11mpg) every day. While it might not look good, that will save miles on the Jeep and allow better off-road mods.
Assuming you wouldn't otherwise purchase such a vehicle, explain to me how you think buying a newer, more fuel efficient vehicle will cause you to "break even."
Some sample math... You have a 10mpg Jeep and average 10,000 miles per year. That is 1000 gallons of gas. At 3.50/gal, that's $3500 per year in gas. Now, the Civic Hybrid averages 40/45. Let's use 45 just to prove a point. Say you drive the same 10000 miles/year. At 45mpg thats 222 gallons of gas. At $3.50/gal, that's $777 per year. $3500 - $777 = $2723. The $23,000 initial outlay divided by the gas savings per year = years to break even. In this case, that's roughly 8.5 years, or longer than the average American holds on to a car.
If your argument is "it's better for the environment," then how many resources were used in creating that car you would otherwise not buy and what are the costs associated with disposing of the older vehicle?
Sorry, but that whole "Thats it, I'm buying a more fuel efficient vehicle" argument annoys me because unless you were going to buy a new car anyways, you fail at basic math.