In Slow Leopard you can’t open MKV files, even if you have Perian installed, so here’s the solution. Usual disclaimer: Works on my machine™.
1. Install Perian, if you haven’t already
2. open ~/Library/Preferences/com.apple.quicktime.plugin.preferences.plist in the terminal, or just use the finder.
3. Expand WebPluginMIMETypes
4. Copy video/x-m4v
5. Having WebPluginMIMETypes selected, right-click and paste.
6. Rename that child to video/x-matroska
7. Expand it, then expand WebPluginExtensions and rename Item 0’s value to mkv
8. Close and now QuickTime X accepts mkv files.