I disagree. This sort of device has the very real chance of killing notebook computers.
I don't think that desktops are going away any time soon, for two reasons. First, if you are going to work for hours at a time, a big screen and keyboard re far more comfortable and practical. Second, really significant work requires the horsepower of a full OS (as opposed to apps on this device).
That said, most people don't need a full computer to travel with. They want something light, with an "always on" connection to the cloud. They want to browse the web, check email, play a game, read a book, listen to music. Sure, maybe they can tinker for a bit with a document too.
Will 90% of the world miss the openness and power of a full computer? I doubt it. And god knows that the screen and multi-touch already beats the hell out of most netbooks. I think it's a no-brainer that this thing will be wildly successful, and I do think it could (over time) kill off the laptop market. (Not for developers or hard-core geeks, but for (so-called) normal people.)
But even if for no reason other than a dislike of Apple, there will be people who don't want one of these. There will be competition.
Anyone here on hacker news that is worried about an Apple monopoly in the future because of this should get to work on competing in whichever area they are able, be it the hardware, software, or business side of things.