Enable Internet Sharing
By looking at the configuration and preference files that make up the Internet Sharing abilities of a standard Mac OS X system and watching what happens when Internet Sharing is enabled on the Mac, it appears that…
/etc/named.conf /etc/com.apple.named.conf.proxy /Library/Preferences/SystemConfiguration/com.apple.airport.preferences.plist /Library/Preferences/SystemConfiguration/com.apple.nat.plist /Library/Preferences/SystemConfiguration/preferences.plist /usr/libexec/InternetSharing /usr/sbin/named /usr/sbin/natd
…are at least required and some modification of the conf and plist files will be necessary. The binaries in /usr already appear to be included in the Apple TV, but no way yet of knowing if they actually work.
Enabling this would allow the Apple TV to be used as a very simple software base station (access point) or a bridge from Airport to Ethernet.