Full screen application causes screen saver to not start (Lion)
A workaround would be to assign a Hot Corner to start the screen saver. To do this, go to System Preferences &gt; Desktop &amp; Screen Saver &gt; Screen Saver &gt; 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.)