Is there a shortcut for this, or can I create one? I'm running 12.04.
I believe there's no default shortcut. However, you can pick one.
The below screenshots are from Ubuntu 13.04, but 12.04 should be somewhat similar.
Click on the gears icon on the top right and go to System Settings.
Click on Keyboard Layout.
Click on Options....
Expand Key(s) to change layout and pick the key (combination) that you want to use to switch layouts.