After performing a clean install of OS X 10.7, Spotlight refused to index my markdown files, even with a QLMarkdown plugin derived from the one that Brett says does the trick for him. Following two similar sets of instructions, I was able to get the Spotlight index working again by adding the markdown file format to the .plist file located here:

/System/Library/Spotlight/RichText.mdimporter/Contents/Info.plist

Depending on how you get there, it may be necessary to right click and “Show Package Contents” for RichText.mdimporter.

What I did from there:

1. Note the correct UTI, which for Markdown is net.daringfireball.markdown.

   In Terminal you can get this for any file type by using the command mdimport -n -d1 somefile.ext, which in the present case returns: Imported 'somefile.md' of type 'net.daringfireball.markdown' with no plugIn.

2. Make a copy of Info.plist and then open it with a text editor or Xcode.

3. Add net.daringfireball.markdown as a string in the document content types array (detail below), and then swap out the modified Info.plist with the original.

4. In Terminal, give Spotlight a nudge to have it re-index your markdown files: mdimport ~/PathTo/MarkdownFiles/*.md

My modified Info.plist file looks like this:

...
<array>
    <dict>
        <key>CFBundleTypeRole</key>
        <string>MDImporter</string>
        <key>LSItemContentTypes</key>
        <array>
            <string>net.daringfireball.markdown</string>
            <string>public.rtf</string>
            <string>public.html</string>
            ...

Back in Terminal you can verify that the import works as it should with the command from step 1. It should now return something like:

Imported '/Users/…/somefile.md' of type 'net.daringfireball.markdown' with plugIn /System/Library/Spotlight/RichText.mdimporter.

That’s it. I don’t know why QLMultiMarkdown wasn’t doing the necessary, but as MMD’s papa suspects some issues with it, I’m glad to have Spotlight indexing work independently of it.