CNet News has a story about HopStop, a service that enables you to plan trips on mass transit from your cell phone:
Google Maps, Yahoo Maps, and even the old-school Mapquest are good at giving you driving directions. But they're pretty bad if you're walking, and they can't help you find your way via public transit. But a New York company, HopStop, can do this routing for you.A similar service exists in Austin, Dallas, and Houston. Unlike HopStop, it's in stealth mode. Contact me if you're curious or want to be a beta user.