It may vary from nation to nation, but under US law the relevant act here is the Sherman Antitrust Act, which deals with single-firm anti-competitive behavior.
We'd have to wade through a lot of case-law here, but if you'll permit me the ability to speak without excessive sourcing (most of this can be found supported and sourced on Wiki)...
The courts have made a distinction between an innocent and a coercive monopoly. Where innocent monopolies, as you may argue but I disagree, are what Apple has in the music player space. These are not illegal. It would need to be shown that Apple has conspired in some way to grow and maintain their market position in a way to be detrimental to consumers and other producers.
Now on to whether it is a monopoly or not. This page can be helpful: http://en.wikipedia.org/wiki/Monopoly#Monopoly_versus_compet...
Right now another consumer electronics company could: a) produce a new unit with a high quality software, b) sell it at a competitive price point to Apple, c) create a product very similar to the iPod itself, d) there already exist many other competitors. To me these factors suggest Apple does not have a monopoly but a very successful product.
And I'd submit as noted above, even if you found Apple to have a monopoly, I do not believe it would qualify as coercive under US law.
As noted in this thread, market share is not so much the issue, as barrier to entry.