I've finally signed up for Zipcar. I was worried that the fact that I have not driven under a US license for 3 years would make sign-up more difficult, but it didn't.
Good article this month in the New York Times on Zipcar, the company and the service.
I should get the card in a few days, and then I'll check out the service. In actual fact I rarely need a car except for business trips (which almost invariably I fly to anyway). But, it will be nice to have the option of driving somewhere, especially in the summer.