I'm not sure whether this is a matter of selling to existing Apple customers (although that's nice, too), and it's certainly not about those who are willing to connect a black box. It's about people who have been pre-sold on the Apple brand but are not computer users (well, beyond reading email and the occasional web page); people who select the free, "regular push-button" cell phone because more than that is intimidating. "Finally, an Apple for you!" And the TV itself, an upscale fashion statement. "Everyone else has a nondescript, black-framed TV. I have an Apple!"
Do you have trouble imagining the ad campaign, showing just how simple it would be to use, how useful, how cool you'll seem to the kids/grandkids? And how you can do even more with it, if only you owned an iPhone...
And, of course, the army of iPod Touch-toting 8-year-olds, who really, really want Mom and Dad to make the next TV an Apple so they can play their Game Center games on the big screen.
IF Apple can make a TV experience that gives viewers everything they already have in terms of program content/subscription services, etc., a greatly improved UI that helps them get more from what they already have and adds cool new features... people will pay a premium - they pay handsome premiums now for TVs that are "better" than the base model, even though the display technology is identical. People who would never consider attaching a black box to their own TV unless the cable guy did it... too complicated.
I think the bigger problem is having the space in today's Apple Stores to display all those big screens. (But consider if the customer could go to any iOS device or Mac in the store and start controlling one of the TVs...)