I think lightroomextra.com should be required reading for all Lightroom fora members.
Anyway, the answer (from Lightroom Extra):
Dual Monitor support - what does LR support?
You can only use a second monitor in the Slideshow module.
You can work around the single screen interface limitation by dragging the right Panels over to the second monitor, but that is it for now.http://web.mac.com/sidjervis/iWeb/Lightroom%20Extra/FAQ.html