This seems like a pretty clear case to me; the Sherman Antitrust Act of 1890 was designed to combat certain types of uncompetitive behavior, where a company would use its control in one market to try and force control over markets that it was inferior in. The result of that behavior was an usurpation of the invisible hand required for capitalism to work; the company was in control where the market should've been.
In this case, it's pretty clear that this is what Apple's doing; they're using their dominance of the smartphone market with their iPhone in order to strongarm their way into a dominance in mobile application development. There's no way Apple can legitly argue that the iPhone is truly a closed system; if it was, then they wouldn't allow ANYONE outside their company to develop for it. Because they have an outright market for applications, it's an open system.
Further, it's also an open system because the ap store spans three markets; you can get an ap from there for not just your iPhone, but also your iPod Touch, and now your iPad. And Apple controls it all.
All told, I'd like to see where this goes. If we're lucky, the courts will order a split in Apple, forcing them to spin-off their Ap Store company, in a move that would yield far better openness for iAps; then everyone who owns a smartphone, smart MP3 player, or slate PC wins, INCLUDING owners of Apple's products. The only loser would be Steve Jobs' massive ego.
Of course, with this happening... Don't be surprised if you hear that Apple starts funneling millions into Palin-2012, in hopes that should a Republican sit in the White House (and DoJ) that they'll step down the legal assault on them. (just like how Microsoft got a reprieve in United States v. Microsoft after Bush took office)