There is a way you can use your NAS to stream content to your ATV2 without Jailbreaking. For the solution to work you will need to have a computer on, running iTunes at all times. I do this at home. My NAS consists of access controlled folders where my personal files/documents sit and an uncontrolled folder where all the multimedia that gets streamed to the ATV sits. In between the ATV and NAS sits an old netbook that does nothing but run iTunes. When I add content to the iTunes library on the netbook (which, incidentally also sits on the uncontrolled folder on the NAS) make sure you add it from the network location (don't map the drive) of the NAS. Set your dedicated computer to auto-boot iTunes when it starts so you don't have to manually open iTunes every time. Link the iTunes library on the PC connected to your NAS to your ATV and your content will either show up in the Computers menu (ATV2) or under respective Movies, TV Shows, Music menus (ATV1).
Incidentally all the PC's in my house share the same iTunes library that's hosted on the NAS so I can access the same content from any ATV or PC.
Of course, if you only have desktop PC's you might want to consider the energy efficiency of this approach. In my case my netbook will for 6-8 hours on a full charge so its energy consumption is not a great concern for me. Such a solution is also dependant on you being prepared to having a device sitting there doing nothing but run iTunes. You could link a desktop PC that other people use but I would not recommend that because if someone using that PC accidentally shuts down iTunes while you're watching something on your ATV the link to iTunes will drop and you will have an unwelcome interruption in whatever it is you're watching.
If you want to link direct to the NAS without having an intermediate device running iTunes you will need to Jailbreak.
Matt.
