More writing against multitasking. Have a look at the commentary too, people attempt to clarify the concepts of what "good multitasking" might be.
There is a difference between doing several things at once (walking and chewing gum) and switching between several activities. In the first, attention is not fully devoted to any one activity. And in the second, time is wasted gearing up for each activity that could be better used on completing one activity.
Recent evidence shows that multitasking is an enormous waste of your time and your company's money.
This references an article in the Wall Street Journal's Work & Family section, New Studies Show Pitfalls Of Doing Too Much at Once (or at WSJ, requires account).