Chromecast has been able to stream movies and music from dedicated apps for awhile but with the new update you can mirror anything on your Android phone. With the next update to the Chromecast app the ability to mirror anything on your Android smartphone. It is in beta right now but look for the 1.7 update in the play store to give it a try. If you have a nexus phone the cast option is built into the settings menu the supported devices list is currently very limited. But others should be following soon.
Source: Google Chrome Blog