The Android SDK tools and ADT plugin for Eclipse have both been updated to version 12. It looks like the big change in the SDK tools allow developers to use system images compiled for ARM v7 and x86 CPUs in the AVD manager and emulator, making consumer sales of Android on Intel hardware one step closer to reality.
The ADT plugin for Eclipse brings a bit more to the table. In addition to supporting the new SDK tools revision, the ADT plugin now allows developers to have better control over APK file packaging, offers better previews and error messages, resize support and RelativeLayout drop support in the visual layout editor.
In short -- this means better tools for developers, making better apps for you and I. Grab the new packages through the SDK manager and Eclipse. For full instructions see the Android Developer website.