Is multi-tasking good or bad?

What about driving and talking on the cell phone? What about answering email while on a Webex meeting? What about using Blackberries at a dinner table? What about…?

Does this increase productivity levels? Can we train ourselves to be better at it?
There is something called cognitive reorientation cost. It was quoted in a CIO magazine article in 2007. Too much multi-tasking is bad. Every time your mind has to switch from one subject to another, there is a transition cost (reorientation cost) that hurts a family or an organization. The more multi-tasking, the more cost. Of course, some multi-tasking is necessary, but we do too much of it.