This setup could be bad if you have several users on the network because this could potentially saturate the wireless network, at least until the ATV loads it's buffer.
From my understanding, but this may not be 100% correct.
Video request and data transfer routing, also why time capsules should have built-in iTunes servers.
ATV to router to Mac with iTunes to router to airdisk to router to Mac with iTunes to router to ATV
Basically this will use a lot of wireless bandwidth until the movie is sufficiently buffered onto the ATV 8GB cache.
Opinion
I think Apple will never release an iTunes server because they intend their giant cloud server farm will take over the duty and allow the ATV to access all of your purchase content from the cloud/net. They won't allow you to do this with your own ripped DVDs because of licensing and they would prefer you to buy another copy from Apple.
Subscription Service
Apple is setting themselves to enter the Netflix and Hulu market at will, if they feel they need to, they will quickly dominate those markets considering their installed user base. This is something that Netflix should worry about considering that some ATV 2 users may have purchased bc Netflix was included. Now the Trojan horse is to have users switch to an Apple subscription service. Just a possibility that should be keeping the Netflix CEO awake at night.