The loss in productivity is not only in the time lost by each task owner, but in an organization, when everyone is in multi-tasking mode, the real hits to productivity come from the delays in completion of projects with dependent tasks. I wrote about this in a previous life as a project managmement consultant… http://www.focusedperformance.com/articles/multi02.html

Despite all the evidence on the wastefulness of multi-tasking, it persists due to 1) the belief that the sooner you start something, the sooner you’ll finish it (only true when single tasking and not bouncing back and forth from effort to effort), and 2) the inability to set priorities, which in turn, is rooted in 3) the deire to avoid conflicts about priorities.