Salon has an article (get a day pass by viewing an ad) about this on-demand, automated taxi monorail system that seeks to address the major problems of mass transit. The big annoyances seem to be that you have to wait a lot, you have to ride with a bunch of strangers and how expensive it is to build mass-transit networks that are easy enough to access.
These guys envision elevated tracks going everywhere, but I think the real deal will happen underground. The more places this system goes to, the uglier it will become.
I also see this system being used to move packages securely directly into the home. I mean, really, why should you have to move 3000 pounds around just to get some milk moved to your house? You do want that to be underground, or this world starts looking really ugly. In any case, FedEx starts looking redundant, as do half of the trips you make on the weekends to Home Depot.
I think that an underground system will actually end up being pretty competitive with air travel and will replace it all together for short to medium trips. No waiting! No terrorists traveling with you. No reservation required. And once you get smaller sections in place, it becomes easier to piece together a coast-to-cost system.
And when robotic tunneling and strong, flexible nano-materials become feasible, we should start seeing a wholesale replacement of the surface streets with an underground network. These underground networks might even be evacuated, making it possible to go extremely fast with no air resistance. Then we can get rid of all of those parking lots.