Bumblebee was a life-saver when I got my Lenovo T520 and first learned about Optimus, but note that link is out of date now. He's forked off that to start again with "Ironhide", while there's another fork that's aiming to maintain bumblebee but fix bugs, etc. Details here: http://www.martin-juhl.dk/2011/08/ironhide-reporting-for-dut...

(If you haven't looked into it, I thought bumblebee was a great hack! As I understand it, it uses the nvidia card for rendering whatever apps you tell it too in a separate X instance -- using the "optirun" command, it doesn't happen automatically automatically -- then uses virtualGL to display the results on your desktop)