Exactly the same things were said of printing - conservatives said that we would lose the ability to remember things when all knowledge could be looked up in a book.
Exactly the same things were said of slide rules and calculators - that we would lose the ability to reason as our ability to calculate atrophied.
The obvious debunking of Carr's argument is the incredible level of skill so many computer users exhibit. Obviously we HNers are all aware of how bloody difficult programming is, but we forget just how complex a lot of very ordinary software is. Watch a skilled Photoshop operator or a really good Starcraft player in action and you'll see Carr's hypothesis go down in flames. Something as mundane as Excel is easily more complex than any system in the world just a few decades ago.
Carr is at the bottom of a very long line of dreary, reactionary trolls, cursing that the young people today don't know how easy they've got it, that they're going soft with their newfangled tools.