Previously in Ubuntu 11.10, when I had Libreoffice open in several workspaces, I would click on the Libreoffice icon to get an exposé-esque preview of all windows from all workspaces. With Ubuntu 12.04, this no longer seems to be the case. One might argue that this is better, but the usage of clicking that icon and picking the right window has become part of my workflow, and I'm very much missing that feature now.
How do I allow the Unity launcher to show all open windows of one application, regardless of their position or workspace?
Ubuntu 12.04
It seems this may no longer be possible, but what may work for you is using Alt+` (grave key) to switch between windows. To get this to show all instances of one program, you have to uncheck viewport bias in CompizConfig Settings Manager. (Install CCSM if you don't already have it)
To remove desktop bias-
Bias alt+tab to prefer windows on the current view port