I saw the open letter from Anonymous threatening to shut down the Phelps gang and the Westboro Baptist Church. It didn't sound right. WBC is not at all reliant on the web, but they are always on the lookout for more opportunities at promoting themselves through the media…so it seemed to me like a futile exercise, with any damage Anonymous might do being entirely tangential to the operation of the GodHatesFags gang, and actually gaining them more notoriety and news.
Cory Doctorow has a more plausible angle. This is WBC itself playing games to draw more attention. He also suggests that the WBC site has been left open as a honeypot to draw in hackers, who would then be traced and sued by the infamously litigious family. I don't know if I'd go that far; I'd give the Phelps' credit for cunning and devious legal acumen, but I haven't heard that they've got the specific skill set they'd need to beat hackers at their own game.