Well, I don't play FR (I did buy the most recent Campaign Setting, but I'm a whore, I buy tons of game material I will never ever ever use in the hopes of cannibalizing it, I even bought The Silver Marches just because Sean Reynolds guilted me into it...and because, as I admitted, I'm a whore) so I couldn't say if it's been revving its engines and heading for a ramp near an aquarium tank.
But Greyhawk hasn't jumped the shark, because it didn't get a chance to. All of the early GH modules (ones like Expedition to the Barrier Peaks, White Plume Mountain, Tomb of Horrors) weren't especially identified with the setting when they came out and are now almost all out of print. The last time Greyhawk got a setting was during the 2E days, and it was never as extensive as the Realms. (I don't count From The Ashes here...maybe I should, but I don't.) I would love to see Greyhawk get more than an assumption of default status in the D&D Core, a lame Gazeteer and some modules. (The Living Greyhawk Gazeteer was better than the D&D Gazeteer produced right after 3e came out, admittedly.) There's been some good stuff put out for Greyhawk...RttToEE comes to mind...but I'd really say that it hasn't had a chance to jump any sharks as yet.
However, if I had to pick something, I'd say the destuction of Rel Astra was a Shark Jump just for the big flashy noise and lightshow value, although I don't dislike that event. I also don't dislike the episode of Happy Days where Fonzie makes that piscean dare, either.