Chris Foresman, over at Ars Technica:
Apple’s approach in the new 15" and 17" MacBook Pros differs from Optimus in two key ways. The first is that the switching is all handled automatically by Mac OS X without any user intervention (though there is actually a System Preference to deactivate it, if you choose). Apps that use advanced graphics frameworks such as OpenGL, Core Graphics, Quartz Composer or others will cause the OS to trigger the discrete GPU. So, when you are reading or writing Mail, or editing an Excel spreadsheet, Mac OS X will simply use the integrated Intel HD graphics. If you fire up Aperture or Photoshop, Mac OS X kicks on the NVIDIA GeForce GT 330M.
Optimus is pretty cool. But Apple building their own system is even cooler. I am envious of this — I’m usually in the wrong mode while using my Mid–2009 MacBook Pro, and logging out to switch is just plain annoying.