Previously, users had to manually select the desired graphics controller in the Energy Saver preference pane, and a logout (or restart) was necessary before the new setting came into effect.
That was less than ideal, and automatic switching sounded like a welcome improvement.
The problem is the way Mac OS X determines which controller to use. According to Snapz developer Matt Slot, "If any active applications links to the high performance graphics libraries (OpenGL, OpenCL, CoreImage, etc), the system automatically switches to discrete graphics mode until that application quits."
This means running various relatively undemanding applications that use these APIs will activate the Nvidia GPU, leading to a significant drain on the battery.
Which applications cause this problem? See page 2.
This is a particular problem for applications that are typically left running in the background, such as Skype, PathFinder, and Yahoo Messenger.