There's nothing really new with multitasking, it's just got a cool name. In black and white days we used to get distracted by phone calls, corridor conversations, smoking breaks or listening to the radio. Today we've got many more distractors but the problem is how well we can concentrate on more than one thing at a time. I admit I like to have several applications running on my computer while I'm working (e-mail, Skype, Twitter, web radio, several web pages ...) and if I'm trying to write something I don't really want to do I've got lots of reasons to avoid doing it. However if I need to really concentrate and think clearly all the distractors have got to be shut down.
Multitasking is often seen as a symbol of being modern and flexible. Interestingly we often see multitaskers as highly efficient whereas in the past they may often have been dismissed as not being able to concentrate on the task in hand. An article in The Huffington Post, Study finds people who multitask often bad at it, describes a new piece of research that suggests that people who like to multitask are actually not very good at it and this leads to errors and carelessness. Indeed the more media they use the worse they perform.
However the report suggests that further investigation is needed into the reasons for multitasking and whether it is a natural reflection of our personality. Some people have a talent for detailed and thoughtful analysis and to do this they shut off all distractors. Others, the multitaskers, are inquisitive and enjoy experimenting and are seldom content to concentrate on on matter at a time. When is the multitasking approach fruitful and when is it not? For the full study see Cognitive Control in Media Multitaskers by Eyal Ophir, Clifford Nass, Anthony D. Wagner (Proceedings of the National Academy of Sciences of the USA).
Another angle on the subject can be read at CNN.com, Drop that Blackberry! Multitasking may be harmful.