I see this with .tex files. I figured it's because TextMate claims various types without declaring a UTI; hence, .tex files aren't recognized as conforming to public.plain-text, so aren't recognized by Spotlight or Quick Look.

The same thing happens in 10.4, when you're using the column view in the Finder, and there's the preview icon on the right. Testfile.txt displays its text in the preview, the same file renamed to Testfile.md doesn't.

Eric Abrahamsen