The keyboard shortcut to change lenses in 12.04 has changed from being Tab to being: Ctrl+Tab and Ctrl+Shift+Tab. As answered in this question: Keyboard shortcut for switching lenses
I can see that this was probably done to improve navigation as you can now choose which direction you want to navigate through the lenses, however I'm a bit stuck in my ways and would like to know if the behaviour can be switched back so that just Tab switches lenses.
These key combinations are hardcoded into the Unity plug-in for Compiz and are not exposed as options that can be changed through a configuration setting. To change them, you would need to recompile your own version of Unity.