I found that PS3 (and other players) now support XSUB streams for subtitles - see http://en.wikipedia.org/wiki/DivX#DivX_Subtitles_.28XSUB.29 It would be great to add support for on-the-fly XSUB addition from .srt or .idx/.sub files - probably via transcoding options. It basically means to create additional stream holding the subtitles…
If you transcode the whole video, you can probably do this using VLC and an appropriate transcoding script.
However, with transcoding you lose seeking support, so I suggest to insert the subtitles outside of MediaTomb before adding the media.
That is something I do not want to do - I have a lots of movies with subtitles in .srt/.idx+.sub that I would like to be visible when viewing on PS3.
AFAIK VLC does not support adding XSUB. Now that I think about it, this should probably be an enhancement request for VLC or mencoder or …