Here seems to be a definitive answer from Nvidia:
http://nvidia.custhelp.com/cgi-bin/nvidia.cfg/php/enduser/stdadp.php?p_faqid=2243&p_created=1224019013&p_sid=SxxZcpjj&p_accessibility=0&p_red irect=&p_lva=2243&p_sp=cF9zcmNoPTEmcF9zb3J0X2J5PSZwX2dyaWRzb3J0PSZwX3Jvd19jbnQ9M SwxJnBfcHJvZHM9MCZwX2NhdHM9MCZwX3B2PSZwX2N2PSZwX3NlYXJjaF90eXBlPWFuc3dlcnMuc2Vhc mNoX25sJnBfcGFnZT0xJnBfc2VhcmNoX3RleHQ9bWFjYm9vaw**&p_li=&ptopview=1
If the link doesnt work this is what it says:
NVIDIA Hybrid SLI® support on Apple Macbook Pro
Question
Does the Apple Macbook Pro (Late 2008) support NVIDIA's Hybrid SLI® technology?
Answer
No. NVIDIA Hybrid SLI technology for notebook computers allows a motherboard GPU and a discrete GPU to work together for extreme multi-GPU SLI performance when needed (called GeForce Boost mode), or use just a single GPU for low power consumption and long battery life (called Hybrid Power mode).
Apple's Macbook Pro (Late 2008) does feature both the NVIDIA® GeForce®9400M motherboard GPU for everyday computing and the NVIDIA® GeForce® 9600M GT discrete GPU for high graphics performance. You can switch between the Geforce 9400M motherboard GPU (called "Better Battery Life") and the Geforce 9600M GT discrete GPU (called "Higher Performance"), but you cannot use both GPU's at once in this implementation.
Apple's hybrid graphics technology is supported under the MacOS X operating system version 10.5.5 and higher only. When running Microsoft's Windows XP™ or Microsoft's Windows Vista™ using Apple's Boot Camp, the system locks into higher performance mode which uses the Geforce 9600M GT discrete GPU for all graphics related tasks and can not be changed to use the Geforce 9400M motherboard GPU for battery life mode.
For more information on Hybrid SLI technology, click here: Hybrid SLI
For more information on the Apple MacBook Pro featuring NVIDIA technology, click here: Apple MacBook Pro