After searching the Internet for days trying to figure out how to get my Windows 7 Ultimate 64-bit system to display MKV thumbnails, I ran across an incredibly simple solution:
The codec pack not only lets Windows Explorer (yes, even 64-bit Explorer) display thumbnails, but will enable MKV playback within Windows Media Player.
There are many other solutions for getting Windows to display thumbnails for other file types. A pretty clever tool is called Mystic Thumbs that generates thumbnails for lots of image and data file formats. It's worth trying, but of course won't help with MKV thumbnails.
This is a fairly short, direct post, but I wanted to get this crazy easy solution out on the interwebs in the hope that it will save others hours of wasted attempts and unnecessary software installations.