Something I thought was cool is that the concept has met with no resistance from the taxi company with exclusive rights to service Sea-Tac, whose representatives said "If people want to be creative and save money and be more green, we're absolutely OK with that."
The New York City Taxi and Limousine Commission is on board with taxi-sharing, too. In July of last year the "Commissioner's Corner" column addressed the different ways the Taxi and Limousine Commission was supporting taxi-sharing, including pilot programs to develop multi-fare meters, support group riding at reduced rates from taxi stands at points on busy corridors, and place "livery stands" at places like shopping centers.
In U.S. cities, where taxis usually supplement public transportation systems, cab-sharing is a great way for consumers to save money while, in many cases, generating additional revenue for the cab driver. In other parts of the world, taxis actually are public transportation. When I was in Amman, Jordan last year, we went everywhere by cab--there is no public transportation, and taxis are super cheap and always available. And according to Wikipedia, dozens of other countries use taxis as the main form of transportation, calling the service everything from "multi-hire taxi" to "taxibus."