I stumbled upon an interesting webpage:
In defense of Westboro Baptist Church pickets.
Basically, the argument this person has is that as long as they're not being violent or endangering public safety, which as far as I can tell has been the case with Westboro, then clearly the 1st Amendment applies and you must defend it. In general, this seems reasonable to me.
Probably the best bet is to just ignore them -- the entire group smells like an ignorant, whiny attention-seeking whore desperate for some sort of limelight. All I have to say is that I am looking forward to when karma has their day with Fred Phelps.