Y Combinator (respect) has an interesting POV on how the iPad is meant to (silently) exterminate Windows from the marketplace:
… (coders) may never want a computer they don’t control, but ordinary people just want something cheap that works. And that’s how the iPad will seem to them. Many will never make a conscious decision to switch. They’ll get an iPad as well, then find they use their Windows machine less and less. When it dies they won’t replace it.
As always, the rest of the RFS is a good read – you may not agree with it but will certainly respect the view…