As a former ][GS owner (a 'Woz' limited edition no less!) this post strikes a nerve in a good way
I'm a very vocal proponent that the iPad is the first device (okay second if you count the iPhone) that will reshape computing as we know it. The reality is that with a *little* bit more functionality around the edges - some of which is already coming e.g. multi-tasking - then the iPad could replace a computer for an extremely large % of the overall market.
Yes, there will always be a need/market for 'real' rigs for coders, engineers, harware tinkerers, etc. Normally I'd add high end video editing and photoshop type work, though I think down the road the iPad and its ilk will be more than powerful enough to do these kinds of things.
Add printing, slightly better document/file handling and sharing within the app structure, and boom.
The question I ponder is what takes over from touch (as touch will take over from mouse/keyboard). Voice? Possibly. Displays: projected, hologram, heads up, in our minds?
20 years is probably long enough that the first wave of the next great cycle of products will have been introduced to replace touch.
As far as this iPad in 20 years: will wifi and/or 3G still function? i.e. will connectivity be backwards compatible? Hmm...