Since Apple doesn't have anything resembling a monopoly in any market it operates in, and since all DoJ can do is file a suit that is predicated on them having one, this is unlikely to go anywhere.
Depends on your view of what a market is. Apple's app store is pretty dominant in the market for "mobile apps".
If i have an xbox, I can get a game from Best Buy or Amazon, etc. Even though those are closed systems.
If I have an iPhone, I can only get apps from the Apple store.
I'm not positive if this is unique to Apple in the mobile space or not. I know I can get blackberry apps from anywhere, same with WinMo. I'm not sure about Android or Palm.