Google Transit, the public transit feature previously available only as a Google Labs feature is now completely integrated into Google Maps. Driving directions will be the default setting for Google Maps but a new option called “Take Public Transit” will help users plan their trip via public transportation in select areas.
What all cities are covered as of today you can find out here.
For a quick reference Google transit is available in California, Nevada, Texas, Oregon, Virginia, Pennsylvania, Washington, Florida, Hawaii and Minnesota. Below is a sample trip screen capture.