There are claims you can netboot an AppleTV via a USB stick (no hardware modification or voiding of warranty necessary). There are other claims that AppleTV running Linux with nVidia drivers can decode HD running the MythTV frontend. Anyone have an AppleTV and are willing to see if this can be done?

Linux on AppleTV: http://www.mythic-beasts.com/resources/appletv/
Boot from a USB stick: http://www.mythic-beasts.com/resources/appletv/mb_boot_tv/stick.html
The Bootloader: http://www.mythic-beasts.com/resources/appletv/mb_boot_tv/

Claim for AppleTV HD playback under MythTV: http://www.silicondust.com/forum/viewtopic.php?t=3722

-Nathan