Will be interesting if anyone comes up with a "Monopoly" or "anti-competitive" class action lawsuit. It's hard to compete in the marketplace with an entity that can charge you $85 for WinRT while their cost is Zero - yet you all have similar hardware costs. The $85 per unit cost savings places MSFT at what some would call an unfair advantage.
Now, if MSFT were the sole purveyor of these WinRT tablets, one could argue that they are merely another Apple type of supplier. But, unlike Apple, MSFT is selling their OS to tier suppliers such as Dell, HP and others.
Suits under the Sherman Antitrust Act have nothing to do with profit margins...only market share. While loosely defined in the Act, subsequent interpretations suggest that you need to have 2/3 of the market to be considered a monopoly. MS is a LONG way from having that. Indeed, I doubt they will ever have that kind of share in the tablet market.