Dug out an old copy of the New Scientist – 24th June 2006 (no. 2557, p46-49). Included a mention of some research from 2005 which showed that dealing with emails and phone calls while working has a greater IQ lowering effect than smoking marijuana. Can read more in this press release from the original report.
The 2006 article (see online article here) is about technology in development which seeks to help reduce interruptions, but also includes some practical advice such as:
If an interruption is likely to take longer than 2 minutes, add it to your to-do list and go back to what you were already doing.
It certainly seems that the current generation of office workers perform worse when they attempt to multi-task.