Unlike the iPhone OS, Mac OS X is totally "open" (and will continue to be, fortunately): the software released for it does not require Apple's approval, nor can its functions be limited by it. that's why we can have tools like Cody Krieger's gfxCardStatus.
This little program aimed at owners of the latest MacBooks Pro, with Intel Core i5 or i7 processors and NVIDIA GeForce GT 330M chipset allows users to monitor and manually switch the GPU enabled by the laptop.
GfxCardStatus runs on the Mac OS X menu bar and informs the user via Growl notification when the chipset is changed automatically. It requires Mac OS X 10.6 or higher and costs absolutely nothing.
(via Ars Technica)