An interesting (or should I say alarming...) article which I just came across. A recommended read for anyone who's a multitasker (yeah, like me 
).
Highlights:
But switching between tasks wastes precious time because the brain is compelled to restart and refocus. "Each time you have this alternation, there's a period in which you'll make no progress on either task," says David Meyer, Ph.D., director of the Brain, Cognition, and Action Lab at the University of Michigan. "It's mental dead time." The result: It takes longer to finish any one chore, and you don't do it nearly as well as you would had you given it your full attention.
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Finally, the stress of multitasking may also impair your ability to concentrate. "We've so trained ourselves to be ready for interruption that we're not paying attention to anything," Dr. Hallowell maintains. "The two things that people think they have these days-but don't-are attention deficit disorder (ADD) and Alzheimer's disease. They're just overloaded from a severe case of modern life."
Link: http://lifestyle.msn.com/MindBodyandSoul/PersonalGrowth/ArticleGH.aspx?cp-documentid=2075672