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?
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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.