I've been unemployed for the past few months and I've been looking for jobs. I plan on going car-free very soon, but I'm keeping my car right now just so I can say that I have a car in my interviews. Every interview I've been to has asked me if I have my own car, and I'm not one to lie. I knew that if I said "I don't have a car, but I plan on cycling to work everyday," they wouldn't hire me.
The last place I went into for an interview is seriously a little under a mile from my house, but they still wanted to make sure I have a car. As soon as I get the job, (if I do,) I'll put my car up for sale. I don't think they'll ask me any questions as long as I'm showing up for work.
If anyone asked me, I'd say "It saves on gas," "it's an easy way to stay healthy," or Newspaperguy's "I like riding my bike more than driving cars." That way, you don't even have to tell them that you don't own a car. It does save on gas, whether you own a car or not. They'll probably just assume you have a car with those answers.
After you've worked one day, I doubt they'll ask you about your transportation unless you call in every time it rains or make a big deal about it.