A friend just pointed me to http://www.twitchhiker.com/ (see twitter feed at http://twitter.com/twitchhiker)
She says "Interesting and fun, but wouldn't happen in large scale."
I'm not so sure though. I think something like this (minus the free tix from airlines for publicity) could become a common way to get around in the future, with the right tool in place, accentuating the right things for people.
What do others think?