I think this is an interesting topic.
There's some fear that if any one company grows too powerful, that it will eventually abuse that power. You can see that meme in a lot of fanboys on the "other side."
I would hope - and it may just be a hope - that as Apple grows larger that it avoids those and other similar pitfalls and is overall more benevolent than malevolent. Unfortunately, history tends to show us that over time there is decay in the organization and that cracks can appear very quickly.
We also see that good organizations go bad, and if they're smart, they get good again. We've seen this in Apple before (think Sculley), as well as hosts of other companies that managed to trap lightning in a bottle, at least for a while.
Much of this will depend on how Job's successors perform. If they maintain the vision and ideals, then things will continue to grow for quite a long time. If not, things to go to hell in a handbasket quicker than you might ever imagine.
In the short term, I think we could yet see a doubling or tripling of market share by Apple. "Short term" here is 2-3 years - I try not to think in quarters unless I'm forced to.
The iPhone and iPad are going to have a continued halo effect, and not just on personal use. No, it will extend into the business and IT sectors as well. There's some evidence of IT allowing a "bring your own" mentality to the desktop, if that continues into a significant trend then Katy bar the door.
