Just did a "fresh" install of Precise (12.04) over a lucid LTS box. It's a Dell C521 with Nvidia graphics (which I think may be part of the problem). I'm not sure which driver I'm using , so information to locate that would be welcome. as it stands I'm stuck looking at the desktop and navigating from there to applications - more likely clicking a document to launch the associated app. the Unity 2D fallback works okay , but I'm pretty frustrated about this install. I guess what I really wanted was to keep all my users, but ditch the programs, drivers, interface for something new (latest LTS). even the users aren't linked properly, but that's a different question it seems.
If it helps, when I first booted up I got an error stating that I should upgrade 'compiz-core'.