I wouldn't think games would be the primary, or at least sole, driver for an AppleTV version of the app store. I'd like to see informational widgets for it. A dashboard, if you will. Weather, movie times, news tickers, stock watchers, eBay auction watchers, shipping trackers... etc etc etc.
Ya, games would be cool, if they figured out a better control scheme than the included IR remote or a $200-500 iPod. Does the ATV have BT? They could do something there. If not a usb-to-radio adaptor and a wireless game controller could be done for under $30. I think the only real market for something like that, though, we be emulated classics. Since they couldn't legally emulate any Nintendo hardware, or the older Sony gear, they'd have to settle on buying up Sega and emulating their various systems. Would be neat but hardly worth the effort.
Anyone who wanted "real" gaming hardware in the living room would just buy a Wii/360/PS3, or a used GC/XBox/PS2 and be done with it. Casual gamers probably aren't likely to buy a gaming addon, or an expensive iPod, so there's just no market for it.
BUT, porting over the Dashboard and letting people write widgets would fit sort of between the App Store and the way the ATV is now.