Perian is a great utility which adds codec's to QuickTime's arsenal, allowing you to play more file types with QuickTime. A fan of Perian has created a simple applescript that will get your Mac to show a nice QuickTime icon instead of a blank icon for the following file types: FLV, AC3, MKV, MKA, and VOB.

Well done!

You can snag the applescript here to fix your icons.