Ironic that a supposed defender of free speech is doing this. Although I find Phelps abhorring, I still support his right to spout his ideology.
There's free speech, and there's hate speech.
EDIT: It should also be noted that in the United States (where Phelps is based), restrictions on "hate speech" have not fared well in the courts, as they are usually seen as an unconstitutional restriction on free speech.
http://en.wikipedia.org/wiki/Hate_speech#United_States
Looks like the Supreme Court is due to rule on a case involving Phelps that may settle some of these questions from a legal perspective: