Just a little heads up. If your ATV2 doesnt show subtitles and the video is coming from a NFS share then permissions might be the problem.
Easy way to check is to look in the folder where the video is located. If there is not subtitle file there, then the problem could be lack of permission for XBMC to write the subtitle file into the folder.
It would be more helpful if XBMC could show that it could not write the subtitle.