I've never used a Zipcar, because it's not available here, but it seems like it would be ideal for bicycle commuters. If you don't drive your car very much -- let's say less than 2000 miles a year -- the Zipcar would probably be a very reasonable way to travel those miles. Hell, taking cabs would be cheaper... when you consider all the costs of keeping that hunk of metal sitting in the driveway (not to mention the fact that you also need a driveway when you have a car...)

My family of 3 has a car and we put about 4000 - 5000 miles a year on it. Personally, I rarely even get in the thing and then it's only because someone needs to be picked up or dropped off...