Multi-tasking messes with our brains in several ways. At the most basic level, the mental balancing acts that it requires—the constant switching and pivoting—energize regions of the brain that specialize in visual processing and physical coordination and simultaneously appear to shortchange some of the higher areas related to memory and learning.
Certain studies find that multi-tasking boosts the level of stress related hormones such as cortisol and adrenaline and wears down our systems through biochemical friction—prematurely aging us. In the short term, the confusion, fatigue and chaos merely hamper our ability to focus and analyze, but in the long term they cause (our brain) to atrophy.
That buzz you get from Twitter-ing, Facebook-ing, RSS-ing and blogging, all while on Skype and rearranging your iTunes playlists is just your brain telling you to slow down. Sometimes being a Luddite geek is a good thing. :-)