Disregard my other answer, it's incorrect.
There are laptops out there that have a physical button for video output switching hardcoded to send Win+p. This is why Ubuntu now interpets Win+p as a video output switching key.
This is hardcoded in gnome-settings-daemon and can be disabled in gconf by setting
false. This workaround probably has a nasty side effect of breaking your real video switch key, if you have one.
P.S. This feature is added via an Ubuntu-specific
debian/45_support_new_video_key.patch, but upstream gnome-settings daemon will do the same thing in a different gnome-settings-daemon plugin (media-keys). It is therefore likely that this gconf workaround will stop working in Natty.
P.P.S. There's an open bug claiming that stealing Win+p this way is not nice for people who have sane hardware and want to use it for something else.