I'm pretty certain iTunes' search used to also find stuff in the audio file comments field. My iTunes (v7.4.2 running on 10.4.10 Intel) no longer does. I checked spelling and everything, but sure enough two tracks that have 'Big Lebowski OST' in their comment field don't show up when I search for 'Big Lebowski'.
Since I remembered that iTunes' search is just Spotlight constricted to the iTunes Library, I tried it in Spotlight and sure enough the two files showed up.
Has anybody else seen this behavior? Is iTunes no longer searching comments? Is this the design or just a bug that managed to slip in recently?