Failing Better: "In recent entries of Roger Ebert's Answer Man column—on December 9 and December 23—he tackled the rumor that Jack Benny had an uncredited cameo in Casablanca. While addressing various claims, such as the fact that one Benny fan-site stated that the Casablanca press kit verifies the fact, Ebert responded, 'I think you're right.'
I'd never heard this before, and didn't dig out my old VHS tape to check, but based on a few minutes searching online, I found a newspaper ad placed by a Florida theater that confirms this appearance—because it gave away a free pass to any theater patron who could spot Benny."
Link via News from Me.
Update: The guy in the screen grabs now posted at News from Me don't look much like Benny. Maybe this is another urban legend, though a good one.