new firmware came out on Friday and i'm really happy with it. they sped up SMB decoding so now i can stream 1080p from Windows Home Server (which doesnt support NFS out of the box). it now supports Vorbis in MKV and audio codec support seems to have improved all around. basically, the PCH can now play all of my MKV files. it previously had problems with background noise on some MKV files with AAC, and it couldn't play Vorbis at all. also had some HD MKV files that would not play, but now they play fine with the updated Haali codec.
that'll keep me happy for a little while, but i'm still hoping they end up supporting OGM, FLV, and MMS (all of which are supported by XBMC). but their main priority should be to redo the UI and make it faster. the PCH seriously kicks ass for media playback, but browsing to find your media is painful. its much quicker/easier for me to browse/find media using XBMC.
i think this last PCH update fully retired the Xbox 360s media functionality. its now just a gaming machine. pitiful, i really wanted the 360 to be my entry point for running custom applications in the living room.