Water based paint is the law in SoCal already and will be in NorCal in 2010, I believe. Most everybody in big urban/populated areas are spraying with waterborne paint. I see cars painted with it everyday and feel that the coverage, matching and metallics are far better than urethane paint. However, don't forget that the clear coat is still urethane. Several of the paint manufacturers are working to develop an waterborne clear coat to replace the urethane.
I-CAR has a waterborne class, which I have yet to take. I've heard that it's a good class to take.