Problems Measuring Apps Running in Full Screen Mode on Lion
I cannot measure apps with xScope that are running in full screen mode in Lion. Why?
When apps are in "Full Screen" mode in Lion they occupy the top most layer of the screen, in other words they float above all other windows or layers you see. Because of this, xScope cannot be "above" them and therefore will not work. It's a limitation of how Apple designed Lion's full screen mode and currently the only workaround is to turn off xScope's dock icon. If you don't wish to disable xScope's dock icon, you'll have to exit from full screen mode before being able to use xScope for the time being.
To hide xScope's dock icon, first quit xScope and open a Terminal window. After you get a command prompt, enter these commands:
defaults write com.artissoftware.mac.xScope generalShowDockIcon 0 defaults write com.artissoftware.xScope generalShowDockIcon 0
Then restart xScope and the Dock icon should be gone. From here, you should be able to use xScope to measure applications that are running in full screen mode.
This is a current bug with Apple's implementation of full screen applications in Lion. We've submitted a bug report to them, and are waiting for them to com up with a better fix for the issue. For now you can find our submitted bug report here.