The title is deceiving. What I mean to ask is, is Plex based off the same code as XBMC? It seems to be not really under active development (no hd audio, 4 year old posts in forums). However, my Diskstation has a Plex server package which I installed and then installed Plex on my HTPC to try it. The installation routine looks a lot like XBMC. My media was already there because of the server package on the Diskstation. Any chance of an "XBMC Server" for NAS devices?
Plex Media Server, and clients for iOS, Roku, etc, are all original products of Plex. The Plex desktop clients for Linux, Mac, and Windows are based off of XBMC v10, and that Plex client update was within the last year IIRC.
There are add-ons that attempt to let XBMC use the Plex Media Server backend, with varying degrees of success. Right now, the closest thing for an "XBMC server" would be a
MySQL (wiki) set up. One idea for the future is to improve XBMC's UPnP sharing so that it can replace the
MySQL (wiki) option in terms of resume watching, etc. Thus, it is possible in the future that any given XBMC "client" could become a "server" and have all these features and more. If this happens, that would likely be far into the future, however.