I also have problem with mpeg-TS, though decoders-support is installed.
Am trying to stream it from MythTV using UPnP, and get "video codec not supported".
Tried copying to play locally, same result. Repackaged into AVI file and it works. mpeg-PS works too. So it seems the problem is the MPEG-TS container from myth, not the codec or mime type.

Has anyone been able to stream DVB recordings from MythTV on the N900 please?