Multi-tasking is a bad idea when you're doing tasks requiring "immersion". After being interrupted it takes you a long time to get back to where you were. One of the least talked about reasons why pair-programming can be so effective is that a pair seems to be much more resilient to interruptions than an individual. In other words, yet again, pair-programming is partly about programming, but it's mostly about the pairing.
Jerry Weinberg observed that if you have two task to choose from you don't in fact have two tasks to choose from. You have three. Your third task is deciding which of the other two tasks you should tackle!
Recently, on a train, a man sitting opposite me was reading The Telegraph. An article on the front page about multi-tasking caught my eye. It quoted some research by Professor David Strayer from the University of Utah. It said multi-taskers often end up juggling activities not because they are good at it, but because they are easily distracted and cannot concentrate on the job at hand. And in contrast, the most efficient multi-tasker is the person least likely to do so because they can focus on one thing at a time. The implication is that someone who claims to be good at multi-tasking probably isn't!