Quote Originally Posted by Don Corleone
Aaah, but there was an attack planned for Britain, correct? Personally, I think you're guaranteeing yourself a hatful of trouble if you codify your laws such that inciting violence is only a crime if you're successful.

That being said, I salute you for your Voltaire like absolutist view of free speech. I assume you are okay with yelling "Fire" in a crowded movie theater as an expression of free speech?
That's public disturbance and being an idiot, it has nothing to do with the political debate and as such isn't part of free speech. Remember, free speech protects political statements(yes, bombing a place is political), not whether you can yell "LALALALALA" into the ears of strangers for hours.

Back on topic, I'm mostly concerned with where this may lead. Let's take an example:

I'm reading the paper at work, and it says that Bush has been an idiot again. I say that somebody ought to whack that idiot(both things happen far too often), and we all laugh at the president. However, one of the guys I work with turns out to be an idiot too. And he doesn't like Bush either. So, at that moment, he decides he should whack Mr. Bush. He kills him, gets caught, and the police ask him where he got the idea. He says that he got the idea when I said that someone should do it. Suddenly, I'm stuck behind bars for 100 years.

That's not a world I want to live in.