In an ideal world where everyone has the best of intentions it sounds like an awesome carpooling idea. But it also has a sketchy aspect to it that doesn't feel safe.
I agree with Jen. It
could be a great idea, but OTOH, it really is not any safer than hitch-hiking, or picking up a hitch-hiker...
It would probably more widely used if there were some safety procedures built in to the system, like registered users ID's have to be verified, and cannot have any violent criminal records, as well as vehicle & driver safety reports, etc....
I would imagine that with gas skyrocketing, and the whole economy going to hell in a hand-basket, a trust-worthy carpool would definitely be something a lot of people in larger cities like yours, would take advantage of. I know my Brother is constantly bitching about finding reliable car-pool's up in Berkeley!