I run Ubuntu 12.04 on an laptop.
I use the laptop screen and a projector plugged in via VGA.
When I run presentations in Libre Impress and click to "run show" (F5), it always shows in my laptop monitor - even if I have the unity bar on the projector screen area and open Impress there.
The same happens with Youtube "full screen" mode.
I'd really like to choose which display "full screen" works on. I can use the workspace switcher to drag the "full screen" window to the other display - but because they're different resolutions it doesn't fit. These full screen modes don't have the usual minimise/maximise buttons.