So it couldn't play the sample videos that were posted?
Yes, Matroska isn't supported out of the box. The media player doesn't even index these files. And apart from it, 720p could be possible with some highly optimized codec, but it won't be easy. I haven't found any 720p video that the N900 plays. VLC and mplayer try to play the 720p videos, but they are very slow and completely unwatchable.