I have read about the Unity HUD.
Is that a mock up or a real thing? Can I install it on my current Ubuntu 11.10 installation or do I need to upgrade to 12.04?
If you are running 12.04, you already have the HUD. Just tap Alt. The HUD is not available for 11.10.
The HUD (Head-Up Display) arrived in early March in the development release of Ubuntu (Precise). So there is no more need to go to a PPA to try it out.
Just press the "alt" key to bring up the HUD.
Ted Gould's page about Searching Menus has some important insights and tips on the fuzzy matching algorithm used by HUD and how it tracks your menu usage. It also notes the fun "hud-cli" command for command-line operation by users of the terminal, and has tips for how developers can make HUD work better for their apps.
Before selecting a menu item you're not familiar with, I suggest navigating to it with the down arrow and paying attention to which icon is displayed to the left of the "Type your command" box. That tells you which indicator or application the menu item comes from. E.g. if you're in the terminal application, press alt and type "date", and select "Date > UTC", you don't just find out what the current date is in UTC. You actually choose the "UTC" entry (if you have one) under the Date menu and change the system timezone to the selected zone....
So even if this is going to be in Ubuntu 12.04, any Ubuntu user with Unity could in theory compile the newest version of Unity, then these two branches, and HUD should work. But the process could be a little bumpy.