Seems like kind of a no-brainer to have a SDK for the AppleTV, with either an accessory controller or using iPhone/iPad/iPod as a controller. The iPhone has an accelerometer that could make it kind of like a wii controller, etc.
I've always thought this.... It's probably something which could easily happen but they are holding back for a product launch at some point.
The App Store is already on the Apple TV, courtesy AirPlay. There's no dedicated App Store because the only "apps" that can utilize a remote with SIX buttons are those that would actually fit the purpose of said store on a television, and the content of said apps hasn't yet been negotiated. So here we are.