yep can confirm on Feisty too. Can also confirm that you can do it without external keyboard. Close the notebook till it sleeps leaving a little space to press a key with something. Then wake it on the external monitor by pressing the key. Then you can reboot and Nvidia works. Can also set fan settings in OSX before reboot if you havent got linux controls yet, they are kept through a reboot.
Heres details on turning off monitor:
from this I'm pretty sure all you need is the computer to reboot with the main display off. I rebooted with both displays on and it didnt work.
Currently installing beryl