Tracy showed me this fun post last month on Get Rich Slowly about the Costs and Savings of Bicycle Commuting. The author does the calculation comparing his/her bike costs/mile to the average costs/mile reported by AAA for car use. The author saved at close to $1,000, and that's even after figuring in the purchase of a $700 bike. Not bad.
It's probably time to recalculate our annual transportation expenses again (though I'm not sure how to count our temporary car situation). I think our expenses have gone down a bit--we haven't had many big bike repair jobs (I can do the basic maintenance), though we continue to have to buy helmets and lights from time to time. We spend a fair bit on Zipcar, but that varies a lot according to the season.