It's not just straight away monopolies as defined by economics that the government sues under the anti-trust act. It is a company or series of interlocked companies such that "they materially reduced competition or tended to create a monopoly in trade"

So if they make an add network targeting say, shopping on their devices, they may get targeted for anti-trust suits since they have the far and away majority of control of mobile shoppers: http://gigaom.com/apple/study-apples-iphone-ipad-account-for...

The fact that iPad and iPhone apps store sales are considerably larger than Android sales is inching closer to the point where the govt can sue about that: http://www.wired.com/gadgetlab/2011/12/ios-revenues-vs-andro...

If there is a dip in android sales for a given year (say, due to maleware problems, version upgrades, etc), then you're quite close to the level where they could get a court to not throw the case out at the summary judgement point. Once you can get a case past there, many people will settle to avoid the costs of a case or the dangers of actually losing (which could be braking up the company).

I think that the figures are still too far off on percentage of app store revenue, but don't be mistaken: Just because there is not an actual monopoly doesn't mean the gov't has no teeth to do anything to apple.