Nick Carr has posted a comprehensive list of sources and related readings for his Atlantic piece "Is Google Making Us Stupid?" This is excellent and worth digging into: "Is Google Making Us Stupid?": sources and notes.
As I wrote before, I think the key question is whether there is scientific evidence for these effects or not — and Carr references one study and a book (Proust and the Squid) that claim such evidence. That's the most powerful part of Carr's article, in my opinion, and I haven't seen a rebuttal that doesn't ignore it (and thus fail as a rebuttal) — to wit, the bloviating, er, debate about this article that continues at The Edge and other such forums.