An interesting article on BBC news:
http://news.bbc.co.uk/1/hi/technology/8219212.stm
As various MT podcasts have said, multitasking doesn't work as well as people think it does. Remember, continuous partial attention = continuous partial lack of attention!
Cause and effect
So basically, people who multi-task only do so because they're bad at focusing on one thing? Sounds intuitively right. (Although I'm aware that this conclusion is several steps beyond what the study actually proves.)