With newer laptops based on the Intel Centrino 2 platform (codename Montevina) switching between a weak, low power consumption integrated GPU and high performance dedicated GPU is possible with laptops with the feature enabled. Some examples are the T400, T500. Note that this feature is not available on any X, W or SL series laptop.
In Linux, switching graphics has to be done in the BIOS. Vista currently has drivers that allows on the fly switching.