CIO Magazine: Why More Is Less:

"The inutility of multitasking as a productivity tool makes perfect sense when understood in terms of attention and available resources. "Current cognitive models suggest that people have a limited amount of attention available at any moment," says Seth Greenberg, a professor of psychology at Union College. "Attention could be thought of as a fuel that can be dispersed. Thus, tasks can be performed simultaneously with efficiency as long as the required attention for both tasks does not exceed the limit." In other words, a person can multitask effectively as long as any given task doesn't require too much attention and thereby exhaust his resources."

01/01/05 update: Get Organized at Work—Painlessly.