In Dconf, schema com.canonical.Unity2d has a key named 'use-opengl'. Under description it says 'Whether to render the graphics into an OpenGl viewport'. But does it make any changes on how the desktop looks, or is it just a performance matter?
For cards that support accelerated opengl such as NVidia, this setting will significantly improve performance when switching between viewports and other effects.
Some older cards do not support OpenGl - and as such this setting should be off. With the setting off, the Raster engine is used - this is a software implementation of the viewport effects you see.
It didn't work in Oneiric - but a fix was released for 12.04.