How do I undo "Scope to this" in Visual Studio 2012?

Click the "Home" icon (third one by default, the one with a house pictogram) in the Solution explorer. This will revert the view to the full solution.

In addition to pressing the 'home' button you should also see, in the top left hand corner of the Solution Explorer, two arrows.

Pressing the left pointing one will take you back out of scope to this and pressing the right one will put you back into it.

Essentially, they work as forward and back buttons.

Extra:

You can also right click on projects and select 'New Solution Explorer Window' to have two separate solution explorers which can browse independently (note that changing the settings on one will effect the other).