Even discounting everything else. The gorilla arm problem will ensure that we'll always need mouse/keyboard computers, unless people are willing to settle with nothing larger than 10" screens. And even 10" is some what pushing it, with the ergonomic problems of holding the iPad, it's fairly heavy for an object of it's size.
Additionally, you just can't ask everyone to happily do all their computing on such a close and controlled OS as the iPhone OS. Where's the unix terminal!
Well the problem is not really the iPhone OS, it's really the same as the desktop OS X, just running a different windowserver. Aka, springboard, and it's capable of just as much as the full desktop OS X. The problem is the artificial limitations placed on it by Apple's policies.
Just how do you think all these iPhone/iPad/iPod Touch/new AppleTV apps are going to be made? They aren't going to be written on an iPad. You'll always need a OS X to create the apps for these products. I wouldn't count OS X out yet. It's still gonna be around for a long time.

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