A University of Michigan study concluded that switching tasks mid-stream actually slows the time of completion of both tasks by 25%, while increasing the chance for error. A separate study conducted by Microsoft found that, when its own workers were interrupted from their main work assignments by e-mail, phone calls or some other disruptive message, it took them of an average of 15 minutes to return to Job 1; and those 15 minutes were often spent on an activity unrelated to the interruption.
Still another study out of Vanderbilt University, through the use of brain MRIs, discovered that multi-tasking actually decreases density in a critical area of the brain, reducing our ability to focus and concentrate. The moral of the story: stick to the task at hand.