Jumping the Shark

What is the deal with this phrase popping up all over these days? Not only did today's PVP strip use that phrase, but last weeks Issue of Ultimate Spider-man also was titled "Jumping The Shark." Of course, the reason that I find it curious is that the phrase "Jumping the Shark" has entered popular ussage, especially considering that it comes from an episode of Happy Days in which The Fonz, jumps over a shark on water skis.