NYT article on the perils of multitasking.  The money quote:

In a recent study, a group of Microsoftworkers took, on average, 15 minutes to return to serious mental tasks,like writing reports or computer code, after responding to incominge-mail or instant messages. They strayed off to reply to other messagesor browse news, sports or entertainment Web sites.

There are some other good studies mentioned in the article.  Worth a read -- if you've got the time.