currently I'm having a kind of very strange problem. When I login as my default user, then unity, whatever I have chosen in start-up dialog, starts with ubuntu-2D. That's exactly what
echo $DESKTOP_SESSION confirms. Also, compiz is not started. When I however decide to login via guest session, unity starts just fine in 3D-mode.
/usr/lib/nux/unity-support-test -p confirms that unity 3d is supported. (Yes, for each line.)
I'm using Ubuntu 12.04 with an ATI/AMD Radeon HD 3200 with proprietary fglrx drivers.
Problem was appearing after I updated unity and compiz. Before everything just worked fine.
What still works is starting compiz by myself, but then I have two side bars, which are overlapping and so not looking fine. I also tried to put compiz into autostart, but then starting my computer is taking much time and I'm still having that overlapping-problem.