Errr...to quote Boundin': That gets a nope. Current iOS developer agreements specifically prevent any payment mechanism other than in app purchase. iPhone devs dance around this by handling purchase in web apps (see: Kindle app). And that trick won't work on AppleTV, as it's unlikely to include a full browser.
You can visualize external subscription mechanisms (a la Netflix), but it's an unwield experience.
This has been the killer for newspaper/magazine subscriptions. Not just the 30% cut that Apple takes, but the lack of sharing of subscription data.
If you're correct then, why should anyone with any valuable media suitable for an

TV want to offer an app? An

TV app store must resolve this problem so that the incentive to deliver content suited for

TV is realized. Otherwise- if you are right such that things stay exactly as they are- only those wanting to give their content away, or those happy to be paid for the cost of the app, will play. That should be a very quick way to kill lots of app upside.
Apple can't be stupid or overly greedy about this. I'm guessing that Apple is getting tired of the lack of interest among the video industry to just roll over in a way similar to their music industry cousins. Apple wants to dictate what their (video) content should cost, and the video players are wanting to price their products as they wish. The music guys argued passionately that all songs are not of equal quality and thus should be able to be priced accordingly. However, they were already under Apple's thumb, and Apple dictated that all songs- be that a #1 hit or the worst filler track ever made- should all cost 99 cents. The video players don't want to end up under Apple's thumb (too), and Apple doesn't have as much leverage to pressure them to accept Apple's demands. Think about it. Should a Citizen Kane, Casablanca, etc be priced at exactly the same price as the rottenest or rotten tomato ranked films?
So maybe Apple has decided that if they want to sell a lot of hardware, they're going to need to approach video content in a different way. Apps would be a way to allow the "greedy" content holders to price themselves out of the market if they so wish. But it would probably also be a better way to allow market forces to find the right price for various qualities of video content. More simply, apps with a way to pay for content through them is much more likely to significantly expand the utility of the new

TV. Provide that feature to content owners, and Apple gets to pitch wide utility of great third-party apps to the base to sell more hardware.
Apple cheerleaders will cast the video content owners as the greedy villains in any comments along these lines. But I wonder what they would do if Apple showed up at their companies wanting to price their company's products as Apple sees fit... especially if such prices were lower than what their company felt was the right price. These other companies want to make money too... not just help Apple make more money.
So, I hope you are wrong in the future when an app store might be announced. If the players aren't going to roll over to Apple mandates via iTunes, an app model (with some kind of in app purchases/subscriptions) is the best way to add a lot of utility & content availability to this new box.