While I can't see giving up a car anytime in the near future for long trips, hauling heavy loads, etc...I have been lessening my dependence on it. No, it's not out of any tree-hugging guilt trips about the environment...it's just simply cheaper.
My move to work at the bike shop included a pay decrease from carpenter's wages, but by lessening my dependence on my tool-laden truck (which isn't a real gas hog anyway), by buying an economical sedan, fixing up a pair of Honda Spree scooters we picked up cheap from the neighbor (the missus uses it to go get the mail instead of firing up the car), and riding my bike or walking, I can afford to live on less.
I could tout the environmentalist party line since I do work at a bike shop, but I won't. As Samurai pointed out, it's a simple matter of economics. We often get customers in the shop that apparently haven't priced bicycles since Hoover was in office (or are comparing to X-Mart's full-scale models of bicycles

), and they often claim that the bikes are too expensive (entry level bikes of shop quality brand names start in the $200-300 range, depending on type). Once you started calculating the actual costs of owning and operating a motor vehicle, the bicycle becomes a real bargain, and pays for itself in a very short time in saved fuel costs, insurance reductions for less annual mileage, health benefits, etc, etc.
Years ago when my budget was severely pinched, I started looking at ways to cut some fat out of how I was living. I calculated my cost of ownership for two automobiles at roughly 25% of my annual income, and this was for cars that were paid for and economical! It made me sick then, but at the time I wasn't willing to make the sacrifices necessary, and the solution was to work more and work harder at trying to make more money. The result was lots of overtime, stress from having to take on jobs for people I didn't want to do business with (but had to because I needed the money), less time with my family and more time spent commuting, more money spent on decompressing from the stress which led to side jobs to pay for it all...vicious cycle.
By simplifying my life, I am now able to do the work I love, live healthier and happier, and have the money that I would've been spending on cars to do better things with...like having fun. If the environment benefits from it, that's just a fortuitous side effect.