The answer depends on the kinds of tasks that are being combined.
For example, there won't be much drop in performance when combining repetitive exercises (this doesn't require much mental focus) with listening to a podcast about complex ideas (this task does require mental focus). But if listening to this podcast is combined with trying to write a challenging essay these two tasks will compete — because both require intense mental processing — and you won't do either task as well as you could if you were totally focusing on it. But will you be more productive with two hours of "podcast (listening to it twice) + essay" compared with one hour podcast followed by one hour of essay? That is a more difficult question to answer.
I.O.U. — Currently this links-page (combining idea summaries with links to web-resources) is in an early stage of development. Later, maybe in early 2012, there will be more ideas and resources here. But what you see below is a beginning:
• A research study in 2001 — Executive Control of Cognitive Processes in Task Switching by Joshua Rubenstein, David Meyer, and Jeffrey Evans — suggests that ultitasking undermines our efficiency: a summary and another summary and more info.
• Multitasking drains brain: Mind is designed to handle one complex activity at a time, researchers find by Sandra Blakeslee, who describes research that "reveals a disquieting fact: The brain appears to have a finite amount of space for tasks requiring attention."
• Memory Problems? Perhaps you are Multi-tasking by Bill Klemm, looks at multitasking and the way our brains work.
• Multitasking — or optimal performance is about mental focus
• Cell Phones and Driving Safety: Wikipedia & Tips for Use & National Safety Council & Utah psychologists (drivers on cell phones are as bad as drunks) so Should motorists ever talk on the phone?
• Advanced Multitasking by Mike Sanders, is about how to do it better by PowerTasking.
• This Is Your Brain Online by Christine Lagorio for CBS, asks: What are the long-term cognitive effects of multitasking (is it efficient in the short term? beneficial or harmful for long-term development?) and the psychological effects of video games (what are the effects on aggression? on mental and physical skills?).