in answer to your questions... (Score 5, Informative)

Hi, I'm one of the engineers at Zipcar. Here are some quick answers to the questions you posted:

Does anyone know what kind of wireless network they use?

We use the CDPD network at the moment. One day, when we get around to it, we'll use GPRS. CDPD is slowish but all our packets are tiny (we use maybe 30kb per month per car), so it's fine for us.

Can they track the car while you're driving it?

For our members' privacy, we don't have GPS in the cars. We can sense the ignition, track the mileage, and keep up with who enters and exits the car, but that's about it. We don't want to know where the members are, and in two years we've only "lost" a car once or twice, so the benefit to us is pretty small. (incidentally, "Do you have GPS?" is the most frequently asked technology question for us.. people think we should have it because it's cool)

Have people been using zipcars, and what have their impression been?

I use it myself and I love it, but I'm biased. See the other replies from our members in this thread.

Is this the future of cars for city dwellers?

We'll see! (I hope!)