I mentioned Nick Carr's Atlantic aticle in a recent post (actualy to point out a different issue). I noted John Battelle's response and now see that Andrew Sullivan of the Times has written about it. While the discussion is mostly annecdotal, there is plenty of discussion online about the related space of multitasking. This Times article - Multitasking less efficient, easier for younger people - reports on research at the University of MIchigan:
When people try to perform two or more related tasks either at the same time or alternating rapidly between them, errors go way up, and it takes far longer—often double the time or more—to get the jobs done than if they were done sequentially, says David E. Meyer, director of the Brain, Cognition and Action Laboratory at the University of Michigan: "The toll in terms of slowdown is extremely large—amazingly so." Meyer frequently tests Gen M students in his lab, and he sees no exception for them, despite their "mystique" as master multitaskers. "The bottom line is that you can't simultaneously be thinking about your tax return and reading an essay, just as you can't talk to yourself about two things at once," he says.
Here's also an NPR article - How Multitasking Affects Human Learning - reporting work at UCLA.