I've been searching 'till I was blue in the face on how to play 1080p MKVs smoothly to no avail. Couldn't understand why my Macbook Pro stuttered. However, I found a lonely, abandoned post on the Perian forums asking me to install avc1Decoder from http://www003.upp.so-net.ne.jp/mycometg3 So all I did was copy the component to /Library/Quicktime and installed the prefPane. Blam, the MKV was playing absolutely perfectly under Quicktime with no hitches! I thought I'd post this here and spread the info. Heck, maybe even a Mac Mini would stand a chance now? My hardware: MBP 2.4ghz with 4g ram under Leopard.