Following up on the answers here from 2013, according to Xavier Ducrohet's response to a question during the Android fireside chat at I/O 2014 (here's a direct link to the video at 46:49), Android Studio is going to be getting the primary focus.
The Eclipse Foundation is working on Gradle support, but this is not in collaboration with Google, and according to Xavier, the support will be more coincidental than Google endorsed. Android tools will not work, but you will be able to write Java, modify resources and build through Gradle once this is released by Eclipse.
It's also noteworthy that Android Studio has now exited preview and is now beta.
I understand all this to be a not-so-official statement that while the ADT plugin for Eclipse will work for the time being, it won't ever be forward-compatible with current and upcoming developments in Android Studio, with the obvious example being Gradle support in Android projects.
2015 update: This answer was accurate in 2014, more has changed since then. While the ADT plugin for Eclipse is still available on the Android developer tools website, the Eclipse bundle is not. There's a warning there that reads:
If you have been using Eclipse with ADT, be aware that Android Studio is now the official IDE for Android, so you should migrate to Android Studio to receive all the latest IDE updates. For help moving projects, see Migrating to Android Studio.
As an additional warning, it reads below that (emphasis mine):
If you still wish to use the ADT plugin for Eclipse, see Installing Eclipse Plugin.