There's another possibility here. Microsoft succeeded because of their focus on developers and, to some extent, having an open ecosystem as long as it suited their business goals. Forgetting this may be Apple's first slip up.
When OS X came out, tons of amazing apps came out that got many people switching: if Apple turns off those same developers, the platform will weaken.
The iPhone didn't get the buzz or the acclaim because of the App Store - it became thought of as a gift from the heavens! Steve has blessed us with this new way to add our value to the closed product. Watch this happen in a different extent with the iPad - something new will come up that will open it up ever so slightly, adding new features where Apple doesn't have the creative or physical capacity to develop.
There are many plausible interpretations of history. Mine is that Mac market share took off around the same time as the ascendancy of laptops, when screens and mobile CPUs got to the point where they were no longer hugely inferior to desktops. Mac laptop sales exploded because they're both more aesthetically appealing (which matters more for laptops since other people see them) and more functional (e.g. sleep and wireless actually work reliably).
I also think early alpha geek adoption of OS X helped to a lesser extent, both in terms of the software they created and in recommendations to friends and family. The iPhone/iPad may become like Windows in the 90s: what developers write for during the day to pay the bills, while they do their hacking on platforms that don't frustrate their desires.