Auto-censors are useful because when people break out the swear words, you get to tell them off for swearing AND for bypassing the auto-censor. Also, there's a good correlation between people who get offended by auto-censors and people who shouldn't be on the internet.
Talking of surreptitious forum censorship, I've got a soft spot for the forum that, rather than banning undesirables, silently ignores 1-10% of their HTTP requests. It's very effective, with minimal drama.
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