What's a tweet, between friends? The law says sometimes it's a threat.

One man thought he was just bantering with his pals when he joked about blowing an airport sky-high. Another was reacting to a radio phone-in when he mused about stoning a journalist to death.

Because they made their throwaway comments on Twitter, both are in legal trouble.

Their cases have outraged civil libertarians and inflamed the debate about the limits of free speech in a Web 2.0 world. The Internet increasingly makes private jokes, tastes and opinions available for public consumption, blurring the line between public and private in a way that has left the law struggling to keep up.

"I think people don't have any idea of the potential legal ramifications of things they post on the Internet," said Gregor Pryor, a digital media lawyer at Reed Smith in London.

"Anything you post on Twitter can come back and haunt you."

Paul Chambers found that out with a vengeance. The 27-year-old trainee accountant was convicted and fined after tweeting in January that he'd blow up Robin Hood Airport in northern England if his flight was delayed.

Chambers - who lost his job and faces several thousand pounds (dollars) in legal costs - said yesterday that he has instructed his lawyers to take his case to the High Court, setting the stage for a major test of free speech online.

"Probably to the detriment of my mental well-being, I am appealing the decision as best I can," Chambers tweeted yesterday.