The Sydney Morning Herald ran a story on the effects of email on a persons productivity and had this to say.
"In a study last year, Dr Thomas Jackson of Loughborough University, England, found that it takes an average of 64 seconds to recover your train of thought after interruption by email (bit.ly/email2). So people who check their email every five minutes waste 81/2hours a week figuring out what they were doing moments before."
There is more to life than email - but I do wonder about all those Twitter and IM interuptions. :)
The related study, at least I think it's the one, is here