Hank Stern at InsureBlog has posted on the risks to car-owners who sign up with car-sharing companies, which coordinate the very short-term rental of private individuals' cars to local people who need to, say, make a grocery run. (These companies should not be confused with Zipcar, which rents out its own cars.)
I strongly agree with Hank's basic point, which is that you should be cautious about insurance issues before signing up with a car-sharing company, because there may be no coverage under your policy if the car is involved in an accident while rented out.
Hank points out that one car-sharing company states that it provides liability coverage to renters, and therefore presumably not to the owners. If he's right then the car-sharing company has demonstrated a shocking lack of judgment, and no one should sign up with it. It is not worth the risk.