You can now edit included layouts not just as standalone layouts, but shown rendered as it appears when included from another layout. 

Let's say you have the following layout:

Here, the button bar on the bottom is actually a separate layout that is just <included> into this page, such that it can be reused in other layouts as well.

Here's the button layout:

You can edit this layout in context if open its context menu - it will list the various layouts (in this scenario, 2) which include this layout somewhere in their view hierarchy:

If you select the other layout, you will see the current layout embedded within the other layout, but with the other layout partially grayed out to make it clear that it is not editable, and to make it clear where the edited layout is:

(Note - this requires the rendering target to be Android 3.0).