tried out some MKV files with the PopCorn Hour
1) HD - it was able to play all of my 720p content, but only a few 1080p files. the problem seems to be a combination of resolution and what codec was used. my tests failed because the content was 1080p and used High@L5.1 with 8+ RefFrames. the 720p content played with the same codec and up to 10 RefFrames.
2) Anime - most non-HD movies would not play at all. checking the forums, this might be because they used an older version of mkvmerge. i'll have to try demuxing and remuxing with a newer version of mkvmerge. almost all TV episodes would play, but only half with audio. the half without audio failed because they use the Vorbis audio codec, which the PCH does not support. there are posts on the forums about reencoding the audio to get these files to play. but i'm going to wait before reencoding. supposedly the PCH team is working on improving FLAC support (recently added with a firmware update), but they are also considering adding OGG support. they are still working on DTS too, so it might be a while.