I've found at least one other topic here regarding this issue, but Apple has yet to address it, or even confirm that it's in fact an issue.
A big portion of my library consists of international artists, many who have special/nonstandard characters in their names (e.g. "Ti?sto"). In previous versions of iTunes, I could search for "Tiesto" (note the absence of the umlaut over the 'e') and all of those tracks would appear. However, after upgrading to iTunes 9, I'm forced to search for "Ti?sto" in order to view those tracks.
Thinking about this issue, it would seem that this is normal behavior, as an artist named "P
Orignal From: Apple, please address this issue!!! ...