This is in Ubuntu Natty:
This is a relatively new development: normally when I watch a movie I hook up my laptop to my projector, close the lid, watch the movie. However, nowadays when I close the laptop lid it shuts off the signal to the projector as well... I've checked the power settings, and it's set to "blank screen" when the laptop lid is closed, but there is no option to do nothing and in any case sanity dictates that the laptop screen should indeed be blanked.
I'm have an nvidia card and have tried setting each display to be the primary one, but this has no effect on my issue.