Whenever I log into a desktop environment, it doesn't seem that compositing is starting. Unity 2D works fine, but when I go to Unity 3D Compiz doesn't start up. I have to type compiz --replace & in terminal every time I login. Another example is when I log into a Cinnamon session, theming support is suddenly gone (although this may be a separate issue). If it helps im using the latest (295.40) NVIDIA drivers.
EDIT: I am having exactly the same issue in both cases, upgrading from 11.10 and with a clean installation, in 32 and 64 bits.