Google Maps integrated the data about public transportation routes previously available at Google Transit. That means you'll have a new option for directions: take public transit. "Google Maps will suggest up to four trips for you to take, which may contain different modes of transportation. For example, one trip may use the train while another uses the bus." You can choose the departure or the arrival time and see estimations for cost and travel time.
Unfortunately, the service is only available in a very limited number of US cities (including San Francisco Bay Area, Portland, Dallas, Seattle) and Japan. There are already some very good trip planners like HopStop or Transport for London, but all of them are limited to a city or an area. Google Maps should increase the coverage and make the transit directions available from the mobile phone.