I just confirmed that Samsung’s TV software is defective; It does not support external subtitles when its streaming movies from a DLNA server.
1) Put a movie.avi and movie.srt in some folder at your DLNA server
2) Try to watch it with subtitles.
No subtitles will appear.. I just confirmed it with Samsung helpdesk. What a shame!
However, the external subtitles ARE supported if you bother to put the movie.avi and movie.srt to a flash disk, walk up to the tv and insert the flash disk. Btw, the TV does not support external hard drives either!