I think that the iPad will be considered the first mainstream tablet that proved the point that meaningful computing does not need expensive parts. Commenters will also note that it is something that netbooks proved first.
But most importantly, the iPad will be seen as the device that opened the floodgate for basic tablets that introduced the internet for billions. I am confident about this. With cheap parts and a basic interface, the elderly can enjoy the web, poor people can have their fun too. With a tablet, you don't have to feel 'techy' to enjoy the benefits.
I almost recommended the iPad to an elderly friend of mine. He used to be the director and the chairman of some respectable companies, so money is not an issue, but he's always had a secretary and he's useless with computers. To check his emails, read his newspapers and e-books, the iPad could be ideal. "Could be", but it's not.
Arthritis is a bitch with the iPad and that glossy screen will drive poor elderly people blind and mad.