When inspecting an element the Web Inspector opens in a new window. The black mask that highlights the inspected element is shown on top of both the Web Inspector and the webpage. Dragging the Web Inspector solves the problem.
Matt, didn't we have a candidate one-liner fix for this?
<rdar://problem/5712796>
Created attachment 19865 [details] Patch + ChangeLog
Comment on attachment 19865 [details] Patch + ChangeLog r=me
Committed in r31128
*** Bug 14458 has been marked as a duplicate of this bug. ***