Full screen application causes screen saver to not start (Lion)
When I am in a Full Screen application mode, the screen saver doesn't start. So I have to shift to another screen that doesn't show a full screen app. There the screen saver starts!
This naturally happens only in Lion.
A workaround would be to assign a Hot Corner to start the screen saver. To do this, go to System Preferences > Desktop & Screen Saver > Screen Saver > Hot Corners… and select "Start Screen Saver" in the drop down menu of your choice.
(Side note: Hot Corners is also accessible from the Mission Control prefpane.)