I just watched this video, "Chance Favors the Connected Mind," from Steven Johnson, the author of Where Good Ideas Come From. (Thanks to Joshua Kim for pointing this out.) I look forward to reading the book.
One thing Johnson mentions in the video already rings true for me: using the Web can damage or diminish some traditional ways of thinking (through distraction, shorter attention span, and floundering) while simultaneously opening up and enriching other modes of thinking (associative thinking, creativity, connecting with others). Those are two empirical hypotheses, and both are subject to investigation.
This is something we've seen again and again. In 1999, I wrote about the gains and losses that came from the adoption of reading and writing long ago, and similar gains and losses stemming from the creation of universities hundreds of years ago. And those tradeoffs resemble the one to which Johnson alludes: more access to ideas and people, which can then lazily mix, while this empowerment simultaneously increase the risks of distraction and floundering.