Here's the thing: cable sucks. I don't want to get a "package deal" that includes channels I don't want. This is an industry Apple needs to reform, and it's going to take more than simply making a set-top box.
Instead of working with cable providers, Apple should go straight to the content creators. Offer an AppleTV with live, streaming television channels you can purchase a la cartè. Each channel costs 99¢ per month, or something similar.
They could have iTunes Store integration ("Want to see this program commercial free? Download now.") The ability to watch TV when I want and how I want would break the strangling hold cable providers have on Americans.
While an Apple set-top box would be cool, it would be awesome to see Apple give consumers a totally new format to watching television.