Watch what you say online...
Be careful what you say online!
Accountant used Twitter to threaten to blow up airport - Telegraph
Interesting case really. On one hand I can see how the comment was a joke. However, it could very easily be misconstrued due to the inherent inability of text to show what the mood of the person was at the time. ie. Imagine that man shouting that angrily down a phone, or now imagine he was a comedian saying the joke on stage. The latter wouldn't lead to an arrest, the first would.
The problem here is that the court would have needed to prove mens rea, a 'guilty mind'. This this could be via 1 of 5 different ways. In this case, they could very easily have shown that a reasonable person could have found his behaviour threatening, showing either criminal negligence or recklessness.
To be honest, you'd have to be pretty dumb to post a comment like that these days.