Apple doesn't have a monopoly as long as there are other portable digital music players out there.
Apple has a monopoly on digital music players the way Honda has a monopoly on motorcycles - they don't have a monopoly, but they own 75% or more of the market.
If someone can not create a itunes alternative then its a monopoly. I still think Sony should buy them out! that would be hilarious to the Religion of Mac! LOL