Windows 8 Cyberlink PowerDVD
So, it's expensive at $15, but the first non-MS video player (and a pretty big name one too) just showed up in the store:
The trial is playing my MKV files perfectly on a Core 2 Duo, so this is great (since that's the format most of my video content is in.) This also includes MKV files with very high bitrate 1080p x264 and DTS audio. I'm not buying it yet, because I'm still holding out hope for a Metro VLC or something, but just thought it was worth bringing to people's attention.
However, it does not support ARM devices yet, and is still a little buggy, so I wouldn't recommend purchasing it yet. Still, it's good to see support for other types of codecs besides h.264 mp4 in Metro. I do also wonder if it's GPU accelerated playback or not. I'd guess that it's not on the older hardware I have but possibly on newer hardware.