You must live in a fantasy land where the people working around you rational adults.

Doing this in the real world will lead people to thinking you are a "dick".

I've used it to good effect, depending on the audience.

The type of meetings that this works in usually involve multiple levels of management and serve more as a forum for peacocking.

Just remark that your presence is not required/useful/necessary for discussion about [the irrelevant subject under discussion] and leave.

That is a great way to marginalize yourself. Better is to have your laptop and say "I'll have one ear in the conversation, and I'm here if you need my input, but otherwise, I've got this SUPER HUGE IMPORTANT thing to crank on." Then you can be excused, in one sense or the other. Of course, it is better still to run the meeting so it happens correctly.

Of course, the first course of action is taking over direction of the meeting when required, but if that doesn't work, a few words with the proper intonation will make the point as clearly as pulling out your laptop. It'll burn some karma, but as a proper slacker you should have enough of it :P.

At my last job, this would either:

A. Add another hour to the meeting. B. Add a second meeting to solve the problem, which I would be required to chair. C. Both A & B.