I agree with KGZotU. I always find the argument that your cost gains will be wiped out by extra food kind of absurd. I mean, you SHOULD be getting that aerobic exercise anyway... and, more to the point, I haven't changed my grocery budget since I began to commute by bike, and I've actually gained mass.
The real money savings come from going car-free, though. No insurance, no license fees, no multi-thousand dollar purchase every few years. If you're going to try and tell me that the single-largest recurring expense most people face is somehow offset by... extra food? You're out of your mind.
EDIT : I can already hear the masses wailing "Oooh, but don't you need a CAARRRR??"
In short? No. Not bloody often. There is a point where insurance costs intersect with rental, if you don't drive often. (That's going to depend on rental and insurance costs in your area, though). If I go to Home Depot and am picking up too much for a trailer, or am going too far to bike, even they'll rent me a truck.
That, or with the right social network I hear you can get rides for the cost of beer. Haven't tried that one, though, as I am a weirdo hermit type person.