Yeah, this could lead to the demise of the cable companies as we know them. For a long time I've said cable TV/Internet needs to be regulated as a utility. With a utility like gas or electricity, the utility company owns the pipes but is prohibited from selling the content that's carried over the pipes. They can set up a subsidiary to sell the content, but they must also allow other gas/electric suppliers to sell to customers at the same transport rates they charge their subsidiary. Those transport rates are set by a public utilities commission. Effectively, the utility company has a monopoly on the pipes (it makes no sense to install multiple gas or electric lines to each house), but due to the monopoly its transport pricing is subject to government approval and it must offer the same pricing to all sellers. Thus maintaining a free competitive market for gas and electricity.
In the U.S., cable TV/Internet has been the big exception. Because it doesn't make sense to install multiple cable lines, most municipalities only grant access to a single cable company. Yet that artificial monopoly is not regulated like a utility - the cable company completely controls the pipes and the content that's sent over those pipes. (This is a necessary step when an industry is first developing. Different companies have to be allowed to try different ways to lay down pipe and offer content over those pipes for the market to determine the most efficient way to distribute that content. But once the best method is determined, the industry is essentially a utility. At this point I think we all know TV/Internet delivery is headed towards fiber to the home.)
If Aereo can get themselves classified as a cable company, that does to cable TV what VoIP did to phone service. Right now the cable companies sell you TV, and oh by the way you can get Internet access too. With Aereo's model, you only need to get Internet from the cable company, and you can get your TV from Aereo. The cable company essentially becomes a utility giving you only Internet service. Companies like Aereo could then sell you TV service delivererd over the Internet.
Unfortunately, this means Aereo is going to have both the broadcasters and cable companies arguing against them to the FCC and the courts. While I hope they succeed like VoIP did, the influence of money in politics makes me think their chances are slim.