Here's a very interesting blog entry that theorizes AppleTV and the iPhone already run OS X Leopard. Furthermore, it offers reasonable speculation on why Apple has delayed the release of HD movie downloads, which finally could be unleashed once Leopard is available and shipping with new Macs...

http://www.blackfriarsinc.com/blog/2007/10/those-waiting-for-iphone-sdk-should

The iPhone has been know to be running Leopard since it was announced. That article is over a month old and now that an SDK has been announced it seems not so much Apple wanted to have Leopard dev tools in place but instead they just wanted to have all hands on deck pushing Leopard out the door. With Leopard shipping next week they can shift more people back over to iPhone development and make the SDK ready for Feb. The Apple TV I think is firmly 10.4 based though, especially since it has been out a while.

Yeah it appears out of the box the Apple Tv runs 10.4.7:

http://wiki.awkwardtv.org/wiki/Apple_TV_OS_10.4.7

I think that the fun with the appleTV will begin once Leopard is finally released. Then it can get its update to 10.5. I'm sure that Apple will want all of its various OS X flavors at parity release points eventually. As has been noted elsewhere, Apple seems to be moving to an OS X-centric model, and away from being hardware-centric. Hopefully that will lead to more diversity in its hardware offerings in the future, and services based on OS-X--like iTunes HD downloads--will proliferate across the various hardware platforms.