90% of the online music bought in the US is bought from iTunes. Since MS's 90% share of PC OS's was considered to be a monopoly, there is a very plausible argument that Apple has a monopoly in the sale of online music.
Monopolies, as we all know, are not illegal. What is illegal, though, is using your monopoly in one area to enhance the sales of other products. Also known as "bundling". MS was found to have committed an antitrust violation by not allowing IE to be removed from windows. This helped IE gain market share at the expense of other non MS browsers.
If Apple consistently changes iTunes so that it will not sync with non-Apple music players, there is a very good argument to be made that Apple is unlawfully "bundling" by using its monopoly in online music sales to increase the market share of its other products.
Legally, this is a very sound argument. But it isn't perfect; here are some counter-arguments, roughly in the IMO order of merit:
1. 90% of online sales doesn't make iTunes a monopoly because only 60% or so of people who buy online music use iTunes. iTunes has 90% of sales not because it is a monopoly, but because iTunes users just buy more music than other users. It is, after all, mathematically possible for iTunes to account for 90% of online sales even if only 20% of online purchasers used iTunes. [Note - I believe I am correct about the 60% figure].
2. Notwithstanding its 90% share of online music sales, the iTunes monopoly is not really anti-competitive because there are low barriers to entry and because users are not locked into using iTunes the way they are locked into using an OS. Pre users still have access to all of their iTunes music. [I think this should be a better argument than it is IRL...]
3. Apple is not stopping people from using iTunes; all Apple is doing is stopping people from syncing their devices in a particular way through iTunes. [This seems like a common-sense argument, but since MS's similar argument (we aren't stopping people from using Netscape, we are just giving them IE for free) was shot down, this one is likely weak.
4. Other - I haven't really worked through this yet; it may well be a decent argument. Technically, iTunes doesn't have a 90% market share of online music sales; the ITMS has a 90% share. While iTunes is the only method of accessing the ITMS, this method is equally available to Pre users, in exactly the same manner. All the Pre users lack is the ability to use iTunes program to get the purchased music to their Pre...but there are many programs for that.
5. iTunes integration is inherent to using iPods/iPhones, and it doesn't make sense to treat it as something different. [History is against this argument, although there is otherwise much to be said for it - IMO the reason that iPods became the dominant MP3 players was not because the device was so much better than the competition (it was slightly better than the best, IMO), or because iTunes was better than competing programs (it was also slightly better than the best, too), but because iTunes+iPod were, together, spectacularly better than the competition - and since you use them together, that's the best way to think of them. But, again, arguing that IE was a part of Windows didn't work for MS, and iTunes early free-standing history hurts it too].
I don't want the first part of this post to be lost at the end of the counterarguments, though - iTunes looks a lot like a monopoly, and Apple is clearly using iTunes to increase the sales of its other devices. Which suggests that breaking iTunes compatibility with the Pre may violate anti-trust law.