Google Maps already shows information about public transportation when you get directions, but now you can see more transit data in a new layer. To enable the layer, click on "More..." and select "Transit" if the option is available.
Google Maps blog lists the cities where the new layer has been enabled and some of them aren't covered by Google Transit yet. "Whereas the main Google Transit product has the goal to provide full schedule information and routing, the objective of the Transit Layer is to overlay lines visually on Google Maps. Think of a virtual metro map on top of Google Maps -- even when we don't have itinerary planning available, we want you to be able to see public transit options that are available."
Here's the full list of cities where you can see the transit layer: Belo Horizonte, Berlin, Bordeaux, Brasilia, Cairo, Capetown, Caracas, Chicago, Copenhagen, Dallas, Dortmund, Duisburg, Düsseldorf, Ekaterinburg, Essen, Frankfurt, Genoa, Guadalajara, Hamburg, Helsinki, Johannesburg, Kazan, Köln, Lille, Lisbon, London, Lyon, Madrid, Marseille, Medellin, Mexico City, Melbourne, Monterrey, Montreal, Munich, Naples, Nizhniy Novgorod, Oslo, Paris, Perth, Portland, Porto, Porto Alegre, Prague, Pretoria, Recife, Rennes, Rio de Janeiro, Samara, San Francisco, Santiago, Sao Paulo, Seattle, Strasbourg, Toulouse, Tunis, Vienna, Warsaw.