Apparently, switching between the two graphics chips is a bit of a chore. You need to open the energy saver from System Preferences, and select either "Better battery life", or "Better performance" under the graphics header. The option seems to be added in a bit of a hurry, as it just kind of floats about a bit there in the dialog. To further illustrate the hackish nature of the feature, you need to log out and in after changing graphics chip. Not exactly the Apple way of doing things.
Some people were getting excited about the rumours concerning Hybrid SLI, hoping that Apple would finally implement support for SLI and Crossfire technology (connecting multiple graphics chips together for better performance), something other operating systems have been enjoying since as early as 1998. Windows Vista, for instance, has no problems dealing with multiple graphics chips, and notebooks with dual graphics chips using Hybrid SLI (for instance the Sony Vaio Z series) can switch between the two GPUs without logging out and in.
In other words, there is still no indication as to when Apple will introduce SLI technology to Mac OS X.