PLEASE NOTE: The features described below revolve around the concept of dual GPU chipsets. As such they are only applicable to the new MacBook Pro range of notebooks. The MacBook and the MacBook Air do not have dual-GPU chipsets.
Apple made a worthy inclusion to its MacBook range via addition of the NVIDIA 9400M and 9600M GT graphics chipsets. However, it is a known fact that Apple has restricted attractive features of the chipset such as the Hybrid SLI -- which would otherwise leverage the power of both the graphic chipsets as a tag-team; furthermore, Apple has also disabled on-the-fly switching between its two graphic solutions -- this requires a needless logout/login process.
So are the NVIDIA chips themselves incapable? Not from what we were told. Further confirmation also comes from a demo shown by NVIDIA to Gizmodo, wherein they confirmed that these graphics chips will soon be heading to different notebooks and also their hardware is very much capable to do all the aforementioned tasks which have been disabled by Apple.The question then is why were the features disabled? Let's take the Hybrid SLI feature -- a likely reason could very well be battery life. Note that the new MacBook's battery is actually lower-specced than the one inside the older MacBooks (the old ones are rated at 5600mAh, while the new ones are rated at 4700mAh). As the notebooks stand, their battery-life isn't that great and it could have been worse under a Hybrid SLI scenario. That could be one possible reason for not including Hybrid SLI. The other -- and this one is more conspiracy theory than plausible -- Apple will enable the feature under Snow Leopard and would like a clear path to encourage users to upgrade to their upcoming OS upgrade.

Of course, it could well be that Apple will enable Hybrid SLI through a free software upgrade program, and in the near future.

As to the login/logout requirement for switching chipsets, the reason we feel is more clear: lazy programmers. Yeah, no reason why this shouldn't have been included right out of the MacBook box. Seriously guys, get working on it already.