From; http://nymag.com/daily/entertainment/2010/06/the_word_you_used_to_be_able_t.html

Louis C.K. was on The Daily Show last night to promote his new FX show, Louie. In addition to making Jon Stewart laugh much harder than he did the studio audience, he explained that there are three words he can’t say on FX, two of which got bleeped out, and one of which is “retarded.” This puzzled both C.K. and Stewart, and it puzzles us as well — we know FX did not always have a ban on the word, as evidenced by the October 2007 episode of It’s Always Sunny in Philadelphia, “Sweet Dee Is Dating a Retarded Person,” which, you can see for yourself, was not operating under any linguistic strictures. Perhaps this very episode inspired a letter-writing campaign that forced FX to institute a new policy? Or perhaps, as Jon Stewart implies, unlike The Daily Show, FX really doesn’t want Sarah Palin to hate it.