Some will play fine with the latest VLC, others won't. Depends on the bitrate and compression of the MKV you're playing. CoreCodec just released their CorePlayer Professional for Mac. Now that is pretty much guaranteed to play all your 1080p MKV's smoothly.
Uh, play them in VLC. Go to preferences, and choose the 'All' setting. Then go to Input/Codecs -> Other codecs-> FFmpeg -> and choose 'Non-ref' for "Skip the Loop filter for H.264 decoding"
I have an early '08 mbp and I can play 1080p mkv just fine. I personally would recommend mplayer over vlc for its superior subtitle rendering and user interface, although you will need a third party build (I use http://mplayerosx.sttz.ch/) or you will have to compile it yourself.