As shared in a blog post, this change is being SRU'ed (Stable Release Update) to flip on the un-redirecting of full-screen windows by default by the compositing window manager. This is another step towards improving the performance and a quick benchmark done by Timo Jyrinki show an 18% increase in FPS.
Up to this point the feature hasn't been enabled by default but had to be manually toggled from the Compiz settings manager. Here is the Launchpad PPA for those wishing to test the new package. There's also ongoing work at improving the Compiz/Unity performance for Ubuntu 12.04 LTS.
See my earlier desktop OpenGL benchmarks as well as more recent Unity tests.