First download and install the
Perian QuickTime codec package, if you haven't done so already. Then download
NicePlayer, and drag it to your Applications folder. Once that's done, launch NicePlayer and then quit it. Now open a new Terminal window and type the following to reload the Quick Look server:
qlmanage -r
Once you've done that, you should be able to open any MKV file in Quick Look. At this point, you can delete NicePlayer if you want, as it's already served its purpose. I've only tested this in Snow Leopard, but I assume it would work for Leopard as well, as NicePlayer only requires 10.4 or higher. On the off chance it doesn't work, though, you can try
this QuickTime hack.