There is a github project called "forked-daapd" that will run on many modern NAS' (some even offer it in their IPKGs... but if not, you can usually find tutes online to get it installed) and features AirPlay functionality.
https://github.com/jasonmc/forked-daapd#readme
With this package, I'm able to connect to my library on my NAS via the Apple Remote app, and then stream media from that library to my Apple TV (3) and other AirPlay devices (like my AirPort express and Mac Mini running AirFoil) just by selecting them in Remote.
Pros:
- none of my computers need to be on
- iTunes does not need to be running
- when playing music, I don't even need my TV on just to operate the ATV, just my receiver
- works with a non-flashed, 100% spec ATV
Cons:
- the interface is not all that great (finding media in Remote is a task-and-a-half)
- media available does not include radio (though it does include PodCasts, etc)
- you need an iPad/iPod/iPhone (which the OP said they had)
- it's not 100% bug-free, and occasionally the library will disappear and streaming will stop
But, for the most part, it's 90% of a viable solution. I use it the majority of the time. Though I do find myself still firing up iTunes from my desktop and streaming from there sometimes... just to get a better interface to find my media.
I agree, a NAS that supports true "Home Sharing" (which would let you stream content directly from the ATV interface, without the need for Remote) would be the ultimate solution. Though I disagree that Apple should make it (or, at least, be the ONLY manufacturer). I would like to see many options.
In addition, I need self-powered AirPlay speakers that I can mount in-wall or in-ceiling. But that's a whole 'nother wishlist.
UPDATE: Sorry... misspoke. forked-daapd offers AirTUNES functionality (which I believe was the precursor to AirPlay, right?). But still works as I described, using the Remote app. Just wanted to be 100% technically accurate. And I do believe ATV 1's & 2's are more of the AirTunes ilk, and might even see the library directly, I don't know.