Any good piece of software uses the OS's file/networking system and doesn't try to implement such protocols itself, so as flopticalcube mentioned since OS X can, Plex can. I use Plex on a Mini as the front end for a 2TB video library which lives on a NAS in another room, mounted to the Mini via SMB. Although you can initiate network mounts directly from within Plex, I do it in OS X, so as far as Plex is concerned they're local.
EDIT: Oh, and to your original points, Plex/XBMC on the Mini is leaps and bounds better than using a 360 for this sort of thing. I have an Xbox as well but I couldn't stand to use it for media playing, mostly because the organization and format support is so much better. Remote support in Plex is great (much better than it used to be), and they even include presets for Logitech Harmony universal remotes
and worked with Logitech to get Plex added as a "device" to the master Harmony database. All in all it's an impressive piece of software. I've
written a few blog posts about my experiences with it.