If you want another solution then using PSM or UMS (and you are using Linux on the system you want to stream from) go for MiniDLNA, it's a wicked small DLNA-server, with some features none of the transcoders have.
Both the mediaserver solutions transcode in stead of stream, so using Showtime to play the transcoded streams on your PS3 in the first place is kinda stupid, it's already a tailored stream for the PS3, so you can just play it from the XMB then. MiniDLNA embeds the subtitles as stream and thus makes em selectable, Showtime understands this, unlike the XMB, also if you hook the opensubtitles plugin you basicly never have to look for subs again.
So far I have not encountered any file that would not play (except h264 level 5+, but those are rare, and can be easily recoded/transcoded)
If I put Ubuntu on the PC that I have all my movies on can I then use MiniDLNA to stream directly to Showtime on the PS3?