That give them a little breathing room before the flood of buyers returning the product in droves because they can't make it work with their televisions.
Apple says it clearly, if not prominently:
"Apple TV works with widescreen, enhanced-definition or high-definition TVs capable of 1080i, 720p, 576p, or 480p resolutions..."But I wonder how many people won't notice that until they open the box?
While TVs that meet Apple's requirements dominate current retail sales, they're far from dominating the installed base of TVs. (See also Scoble's lament that his Dad isn't in the digital TV generation.) If you have an older TV, you're just not going to be able to make the connection.
Mickeleh's Take: There are more than enough modern TVs to make Apple's numbers, but there are also many more older TVs that will make headaches for the returns counter at the Apple store. Apple's delay on shipping the product gives them another few weeks to stock up on aspirin.
Other Mickeleh's Takes on Apple TV:
Pixel-Punchy Scobel's Rant about Apple TV
Apple, Pioneer of Accessible Solutions,
Neglects Closed Captions in Apple TV