For some reason Web Inspector doesn't activate anymore via "Inspect Element" context menu or Develop menu - no console messages or errors encountered. I have an older copy of the nightly Jan 26th build (Version 5.1.2 (6534.52.7, r105964)) and invoking the web inspector is working there. On Safari 5.1.2 works fine as well.
Updated to current release: Version 5.1.2 (6534.52.7, r106466) Not working there.
*** This bug has been marked as a duplicate of bug 77424 ***