had I actually argued that market cap = monopoly. I didn't.
As I'm sure you would agree, Apple has had the luxury of being somewhat of "the also ran" company vs. big, bad, evil-intentioned Microsoft. They've been able to fly under the radar in many respects where Microsoft was always under monopolistic scrutiny (and sometimes for good reasons, admittedly). Now that they are the $$$ king of the hill (which, of course, could change in an instant), flying under the radar isn't going to work. Goes with the territory.
Of course, "also ran" still meant hundreds of billions of dollars to throw around, but hey, what's that amongst friends?
Now, as to your OS monopoly claim, let's see: There's Windows, there's OSX, there's the iPhone/iPad OS (which, in no small measure is directly tied to Apple's fortunes these days), there's Linux, there's Android, there's WebOS, there's Chrome, etc.
As for music, there's iTunes, theres...um...uh...oh yeah...Amazon...maybe Walmart. Don't forget about the Zune!
Yeah, I'd say where monopolies go insofar as real choices are concerned, the digital music distribution channel seems to be a lot more locked up than the OS situation is these days.
From wikipedia regarding the term "monopoly": "A pure monopoly is an industry in which there is only one supplier of a product for which there are no close substitutes and in which is very difficult or impossible for another firm to coexist."
As we compare the state of operating systems in the marketplace vs. digital music sales, which sector would come closer to the definition above? Today, I think it's quite clear.