I would love to have an app store for the aTV, but the problem is that I don't think Apple believes that the aTV user base is large enough to make an aTV app store worthwhile. Personally, I think an aTV app store would drive aTV sales, but what comes first, the chicken or the egg?
Anyways, I would guess that an app store will happen as soon as Apple stops referring to the aTV as a "hobby".
People can already view apps on their TV using screen sharing with the iPad 2, which solves both of those issues (well, the first is ignored). Because of that, an ATV app store could cause a decrease in iPad 2 sales.
Well, going this way is OK for a few things, but for stuff like Netflix and Hulu, it's incredibly wasteful of your network resources. Your iPad would be using both download and upload to your router and the aTV would be downloading. If you had two iPads streaming two movies to two separate aTVs, you'd use up all of the bandwidth of a 11n network pretty fast.
If the aTV had Hulu, etc. directly on it, you'd free up the network resources that the iPad was chewing up.
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