The experimental ART runtime compiler got introduced alongside Android 4.4 KitKat and it's time to shine will come with the next iteration of Android, just as we told you several months ago. The latest changes to AOSP's source code indicate that the upcoming version of Android (be it 4.5 or 5.0) will indeed run ART instead of Dalvik by default. The latter uses a Just-In-Time (JIT) code compiling, which processes code only when an app needs it. ART, in the meantime, comes with an Ahead-Of-Time (AOT) compiling, which runs the code before it is needed. ART usually compiles said code during the installation of a given app, thus substantially reducing the time needed for said app to launch.
While ART offers slightly better performance and improved battery life, there are still some apps that are still not optimized to run under the new runtime. Those that have not been rewritten to work with ART might not work probably and cause a lot of issues. If you are into experimenting, you can give ART a try at the moment, too. Your device has to be running Android 4.4 KitKat or higher.