MythTV has a builtin UPnP server that can serve recorded shows to the PS3's UPnP client. However, the format that MythTV records may not be directly compatible with the PS3s video client.
I use the following user job in MythTV to transcode recorded videos to a format that is compatible for the PS3 over UPNP. My PCHDTV-5500 card captures audio in MPEG-PS and the PS3 will only play MPEG-TS formats (thus the requirement to transcode the video file).
Be sure to change the two instances of "/storage/mythtv" to your appropriate mythtv video directory.
This is script performs the following steps:
Enter the script above as a "User Job" and enable that user job on any recordings (under advanced options in MythWeb) you'd like to be able to view on your PS3.