Having the same or similar problem I just wonna give my 5 cents:
I have two ATV2, where one is connect to my Mac via Ethernet, because this is much faster than Wifi.
When Wifi is on, everything works well over Ethernet (which is has the top priority in Network settings).
But, when disabling Airport, after some time Homesharing breaks and the computer isn't found anymore.
Also, my iPhone4 doesn't see the Shared stuff anymore.
Switching off/on Homesharing in iTunes on the Mac again (with Wifi/Airport on the Mac still off), the computer is found on ATV2 and iPhone4 again immediately, but the connection drops after a while again. It looks like the re-enabling of Homesharing in iTunes does a single 'broadcast' on the network to advertise the computer, so that it gets detected by ATV2 and iPhone4.
For the Ethernet wired ATV2 streaming videos works flawless as long as Airport is enabled, which looks for me that there is a repeated 'broadcast' of the computer's IP on the network.
This looks even more evident for my second scenario where I have a ATV2 connected wireless to an Airport Extreme (5 GHz band). When the Airport Extreme connect to the Ethernet port on the Mac is streaming via en0 with the internal Airport enabled, streaming works really fast until it breaks after a while. After that point I recognized that the traffic now is not going anymore via en0 to the Airport Extreme, but via the internal Airport. When disabling Airport, the computer isn't found at all.
For me the whole story isn't implemented very professional and I have spent days already to come to a satisfying working solution without any breaks.
No ideas! Thanks!