In light of what I'm reading elsewhere about the risks of multitasking, I thought it worthwhile reflecting on what multitasking does.
In the old days, computers did one thing at a time. That was efficient for them, but it wasn't always great for us. We humans began to see the advantages to us of their doing more than one thing at a time. That is, I benefit if I can be printing a document, updating software, and reading my email, all at the same time. In that case, I'm only doing one thing: reading email. I told the computer to print the document, and I'll pick it up later when I walk by the printer. I told the computer to get the latest version of a certain application; I don't have to do anything else, and it will be ready for me when I next use it. I can now read my email without waiting on the other tasks to finish. In addition, the computer may be doing other useful things (for example, getting new email) without me having to even ask. That can be very good for me.
From the computer's standpoint, that can be a problem. The computer has to do more work multitasking than if it were only doing one thing at a time. The good news is that computers have gotten faster, and they've gotten features that help them multitask more efficiently, so they have the capability to keep up with our demands (at least most of the time).
So when someone talks about productivity and multitasking in the same sentence, make sure you know whose productivity they're talking about. If this multitasking stuff works as it has in the past, your multitasking may enhance someone else's productivity, but it makes you work harder. That can only work if you have the excess capacity (time) to get all your work done and manage to switch between tasks efficiently. Did you get that quad core brain implant last week?