So I'm a little behind in my reading, but as I was perusing the December 2008 Planning magazine I ran across a cool idea that helps reduce single occupant vehicle trips, but allows people the freedom of having their own cars.
The article, Travel Smart A Proposal: Commute by Freeway and Save Energy (second item) suggests a system in which small cars are loaded onto an auto carrier (like the ones that haul new cars to the dealership) in the suburbs and hauled down the freeway to the central business area.
This would help reduce the number of cars on the road and the need for large commuter parking lots in the suburbs (thereby reducing runoff and urban heat island effect issues).
Of course, there are some flaws: the concept would work best for small cars, people would need to have the cars turned off during transport to achieve maximum environmental benefit, there would still be need for urban parking, etc, etc. Regardless, it's an interesting concept.