This issue keeps coming up. This is a pretty narrow definition of multitasking and is not really considering things that get parallel. I had an argument with my high-school friend about this, and wrote http://www.ciexinc.com/blogs/daily/080601.html

Now I do agree with Joel that there are certain kinds of tasks that are very expensive to interrupt, but to rule out all kinds of multitasking based on Italian judges or brain scan imagery, no.