there are several types of lg-bd550, it depends where it was purchased. I got mine in the US and it does not support *.mkv playback. However it has netcast (roxio cinema, netflix, vudu, MLB, napster, youtube, divxtv, aacuweather etc)instead. Others have a combination of both or just the *.mkv playback.
Mine is different from Conan wherein the playback keys and USB port are hidden under the bezel. I think the model he has is the same as those purchased in Europe, some areas in asia (Japan included) and the middle east.
For some strange reason features were limited to avoid features (netstreaming) that is not applicable for that region.
However, for the adventurous you can try swapping the firmware. I think the Singapore BD550 offers both *.mkv playback and netstreaming (but for youtube, picasa and accuweather only).