Here's an interesting little video put together by the people over at Cliff about what the effects of using bikes more often for trips under 2 miles would be.
http://www.2milechallenge.com/ride/

I put my address into the 2 mile radius map..... wow, I usually don't bother to pull out the car or the bike for trips that short, until I'm getting the very outer edges, then I might bike. I generally prefer to walk....... I think I must be really odd........