I was thinking of installing GNOME 3 in a regular Natty, but I would like to have the option to switch to Unity available. Since building Gnome Shell turned out being quite hard a task, I was considering installing Unity 4.x, the Oneiric version based on GNOME 3, via one of the avilable sources, such as Launchpad. Would this be possibile and safe? Thanks for your advice!
While this might be possible, I would suggest that you rather just upgrade to 11.10. It would not be worth the trouble (and risk of breakage) to compile Unity 4.x on 11.04, while also trying to upgrade to GNOME3 (which will itself make your 11.04 system much closer to instability).