This comes hot-on-the-heels of AT&T's move last week to discredit the FTC's right to take action against it, referring to parts of the FTC Act to back up its opinion. Given the timing, it seems unlikely that it was that move last week that spurred the FCC on to investigate things, but as consumers, it's a great thing that top-level agencies are willing to get involved. As FTC Chairwoman Edith Ramirez said this past October, "The issue here is simple: 'unlimited' means unlimited". Or at least it should.
AT&T's defense is that unlimited users are hogging a good part of the network -- something that should come as a surprise to no one. It claims that 5% of its unlimited plan users were responsible for utilizing 25% of its network's capabilities. That's a fair argument, but it doesn't discredit the fact that AT&T didn't expose to its unlimited customers that they were going to be throttled - the company instead did it discretely.
One of the biggest issues with AT&T's actions here is that it touted 'unlimited' all over its advertising, emphasizing it so as to lure people in. And once they were in, they'd throttle them if they used up too many resources.
We'll have to wait and see what becomes of this, but at the moment, it seems like it's well-established that AT&T was deceitful, so we'd hope that this means that it, and every other carrier out there, would stop trying to play that game with its customers.