I think one reason people have a hard time visualising how the iPad will fit into their lives is that they assume it will mainly be a mobile device; i.e. something they will actually carry around with them all the time.
I don’t think that needs to be true for it to be successful. I think it’s significant that Apple had a sofa on stage for the product launch.
Despite the fact that there is more than one real computer in the house, I often use the iPhone at home to look things up on the internet or check my email. Because if you just want to do something quickly, the device which is in the same room beats the one which is somewhere else; but also because sometimes it’s nice to do all that stuff from the comfort of a sofa.
Nintendo recently brought out a larger version of the DS, which might not make sense if you believe that a handheld device is all about portability. But it makes sense to me, because I don’t actually use my DS on the train or waiting for a bus; I sometimes take it on holiday but otherwise it doesn’t leave the house. It’s still nice to be able to just pick it up and play it anywhere. And somehow it feels like less of an effort — you can pick it up, play for a few minutes, put it down — it’s a more casual, comfortable experience than using a ‘real’ games console.
I think the iPad could be a successful product just to have around the house, something to pick up and use for a minute or two as an internet device or for casual gaming; something you can curl up on a sofa with. Something that never leaves the house, except perhaps to take on holiday.
Some people will carry them around, of course, and no doubt people will find lots of ingenious uses for the thing — it is a blank slate — but that may not be the norm.