As previewed at Google I/O in the What's New in the Developer Tools talk (jump to around 20 minutes), Multi Configuration editing has now been integrated into ADT 21, and is available in the Preview builds, as of preview 9. (See the ADT 21 Preview Page for installation instructions).
Here's a screenshot of a layout which has many translations, and the preview mode has been set to preview all locales. Click for larger image.
Here's a screenshot of this feature being used to show how the current layout appears when it is included from other layouts:
Here's a screenshot showing a layout rendered for multiple screen sizes:
There are several preview modes available:
As you're making edits, the previews are updated in the background.
You can click on a preview to switch the current configuration to the configuration of the preview.