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, W500. Note that this feature is not available on any X or SL series laptop.
In Linux, switching graphics has to be done in the BIOS. In Windows, only Vista currently has drivers that allows on the fly switching. To do so, you need to have the power manager installed and running (Version 2.41a tested), then right-click on the green battery simbol (not the one from Vista) and choose the high or low power gpu under Switchable graphics.
On Ubuntu Intrepid Xorg wouldnt work at all ( vesa, intel and radeon drivers ). BIOS setting has to be either on Internal or External, not Switchable.