Do you mean setting up a Mac Mini as a Plex media server, and running it 24/7 as a streaming device?
As someone who's only doing research on the topic now (I don't own a NAS, Plex server, or Apple TV), but from my understanding of your situation, you could either plug your NAS into your iMac and reference (i.e. import) all the movies in the NAS into your iMac's iTunes app. That way, iTunes will know that the movie is on your NAS. Even if you were to unplug your NAS for a day (why?), or turn it off temporarily, the movie would sit on your iTunes, but be unplayable. It would still be in your library's movie list, but it would be unplayable. No biggie.
The other option is to install Plex onto a Mac Mini, and follow some instructions on the internet to install it. Having said that, I don't know if Apple TV works with a Plex server, or if it can be MADE to work with a Plex server (officially or unofficially). I haven't looked too deeply into this. If it works, or if you're willing to use a Roku 3 instead, then install Plex onto your Mac Mini, plug your NAS into your Mac Mini, and use the Mac Mini to stream it to your Apple TV (again, if Apple TV is compatible with Plex). If that doesn't work, then get a Roku 3. It's inexpensive and will allow you to do this.
If you could connect your Mac Mini directly to your TV, you can still use Plex, but you wouldn't need to stream video via Wifi. The advantage is that the hardware requirements of your Mac Mini would be much lower, and you could basically use ANY computer to install Plex on. If you're streaming video wirelessly, it's different. You would need a decent computer to stream HD video. Furthermore, it'll depend on your Wifi network speed, coverage, and dependability.
I hope this helps. I'm not 100% sure about what I said, but I'm pretty confident with what I posted. If someone else has something to add, I'd also love to read it.
I'm actually interested in buying an Apple TV today (!!!), which is why I've been doing my research. It's interesting for me too.
I decided on the Apple TV over the Roku 3 because I don't have a NAS, I don't have a Plex server, and I want to be able to play subtitles (mostly .srt and .ass). I think it's possible via iTunes, so I'll try that instead.
Also, I like the idea of using AirPlay, and using the Apple-TV's optical out to connect my Apple TV to my older stereo (no bluetooth or AirPlay). I would just need to buy a DAC so that I can connect my AppleTV to my DAC, and then connect my DAC to my stereo.