Zipcar has been rolling out "one-way" trips in several cities: LA, Philadelphia, Denver, Boston, Seattle... You still have to return the vehicle to a Zipcar parking spot, but it can be at a different location from where you started your trip.
Given that Car2Go and ReachNow (BMW's car-sharing beta in Seattle) seem to be thriving, it seems wise for Zipcar to explore true one-way trips (i.e., park anywhere).