Works the same here as it ever did. I think normal behavior is that the primary monitor (where the dock lives) is the one to play the video while the other goes black.
For those interested, Quicktime Pro 7.6.6. works grand in Lion, and in fullscreen leaves the other screen free to use as you wish. Inmo, this is how new QT should behave, but heck, they want ice water on mars as well.
Well, that's the human condition, we have been doing that since the dawn of time. But I suppose some smart applescript creator could automate that somewhow...not sure if applescript has access to system preferences, but it would go like this:
open qt in fullscreen
@ movie end
change prefs back