Although a few posts have touched on this, I thought I'd highlight media streaming from a NAS (if that is even a consideration for you).
If you want to stream Apple DRM'ed media (protected content you bought via iTunes, basically) then you really can't stream directly from the NAS without a Mac/PC running iTunes, as noted previously.
On the other hand, if you're planning to play your own movies stored on the NAS that you've already transcoded into iOS-compatible .mp4 files, then you want to look for a unit that supports the streaming protocol you decide to use. Currently the two most popular choices are UPnP/DLNA and SMB/CIFs shares.
For UPnP/DLNA you'll need a compatible player such as BUZZ Player HD or MLPlayer. (As an aside, XBMC is a great and free multitplatform UPnP/DLNA player for desktop machines)
Another option is that some player apps will allow streaming over SMB/CIFS shares, so you need an app that supports streaming over this type of fileshare. The tricky thing is that to properly support actual streaming (i.e. allowing your video to start playing immediately w/o having to download the entire movie file to the iPad first, scrubbing, etc) the embedded SMB server in the NAS must support it. How can you tell? It can be difficult, since not all NAS units reveal that type of info. One approach is to research the various media player apps and see if they list compatible NAS devices. Another way is to get the NAS first then just start trying a bunch of apps to see which ones work.
Anyway, hope this helps. I keep a bunch of the family's movies on a homebuilt NAS/media-server and it saves a huge amount space in not having to sync them onto the iPad just to watch.