Correction, If you want the best NAS functionality buy a Mac-mini w/ the HDMI out.
That's not a correction. It's an OPINION. Perhaps you could learn the difference? In any case, a Mac-Mini is a pretty darn expensive "NAS". It's more like a media PC. The problem is Apple doesn't have any way to rent the vast majority of movies from a Mac-Mini or you could use it in lieu of an Apple TV (well you still can minus iTunes rentals).
like i said earlier, unless your NAS supports sharing iTunes libraries natively, you wont be able to look at the movies stored on the NAS.
Sorry, but that won't help either because of the security code check. Since there is no way to enter it from the NAS, there is no way to pair an AppleTV to an NAS iTunes server.
In any case, NAS units are just small (usually headless) PCs that tend to often run Unix or Linux. One's only concern for leaving a PC on should be its overall power draw. Compare what an NAS uses to what a small PC would use with its monitor turned off. I'm thinking a low-powered PC Netbook (some are under $250, which is comparable to an NAS unit) might be ideal for operating as an iTunes server. They're designed to use as little power as possible (most don't even have fans). Set it up with iTunes, plug in a big hard drive and set the monitor to turn off after a short interval. Instant low-power iTunes server. Heck, I'm tempted to test the idea out with my hacked Dell netbook running OSX just to see how well it can serve movies.