Multitasking is hard. Let me change it. "Useful Multitasking is hard". It takes times. And one of the most important parts is your brain in this process.

Human brain doesn’t multi-task like an expert magician; it switches frantically between tasks. In there, real problem occurs when we try to concentrate on the two tasks we are dealing with, because this then causes an overload of the brain’s processing capacity.

Try it yourself. When you try to do more than one task, you may feel your brain performing slower.

Earl K. Miller says;

People can’t do it very well, and when they say they can, they’re deluding themselves. The brain is very good at deluding itself.

Gloria Mark's studies Constant, Constant, Multi-tasking Craziness shows; when people are frequently diverted from one task to another, they work faster, but produce less. After 20 minutes of interrupted performance, people report significantly higher stress levels, frustration, workload, effort and pressure.

And also read this great question Does listening to music help or hurt productivity?

Listening to a webcast needs more attention than listening to music. If music hurts our productivity in multitasking (some of us), listening to a webcast can cause even more attention loss.. I don't suggest it!

You can't do the same kinds of task together, but you can sometimes do 'multi'-tasking as long as it is an entirely different part of your brain...

For example, you can do a task that takes mental concentration, while doing a physical task that you can do on auto-pilot. E.g. Listening to a complex podcast, while jogging. Watching a TED Talk while washing the dishes (I love laptops).

What you can't do, is the same kind of task, or two complex tasks at once. Thinking about a programming task, while trying to pay attention to a webcast. Same-same, see?

You need to adjust the tasks you are multi-tasking, to make sure that you have enough concentration for both. If there's any chance you'll get so engrossed in a podcast that you run off the road, that's too complex. Stick to music instead.

This can be useful, for when you are doing a task that doesn't engage all of your attention. I.e., is boring. Check out this article: http://lifehacker.com/5162029/doodling-increases-focus-and-recall

You may be doing a boring repetitive task, say adding up spreadsheets, and in cases like this, a little bit of distraction, eg music or a really light podcast (more entertainment), can keep you on task without getting distracted. If you find yourself distracted, or unable to concentrate on one task, then one of the tasks is too complex.

I have a little continuum, like: Familiar Music without words - Unfamiliar music with words - Light entertaining podcasts (that you can 'get the gist of' without learning too hard) - Complex learning podcast

Depending on what I'm doing, or how much energy I have, determines which I can listen too.