PS3Muxer is a video file remuxing utility for the Mac. A remuxer is basically a video editing tool that can deconstruct a video file and then convert either the video or the audio, both, or none to make the file playable on a specific device. In this case, the target device is the Playstation 3. The PS3 can't play MKV files, but it can play M2TS files. Those are container formats. You might have an MKV file encoded using H.264 and AAC, but simply because the container is MKV, your PS3 won't play it. So this remuxer can open that MKV file and then save it into an M2TS file, without converting. The main advantage in using this method is speed, since remuxing a file is considerably faster than converting it. In some cases, PS3Muxer will let you convert your audio track if it is not supported. Converting only your audio track is a better alternative than converting the whole file.
In my testing of this app, I didn't run into any problems. I loaded an MKV file, remuxed it, and then played the M2TS file on MPlayerX. The M2TS file looked identical to the source file and the process was fast.
The only bad thing about this app that I can think about is that it doesn't support subtitles.