Thanx!! Discovered that it was actually a data type mismatch that was causing the query to fail, but that link is a great help for any further MS Access SQL questions that may arise.
Thankfully I don't need to worry about the select performance issues as much. The IN() is being used in an update query designed to batch edit multiple values being scanned into the db from the filesystem. Thus, the total number of values in the list is usually going to be very small (< 100). The app is an MP3/DVD/CD catalogue, and the program obviously can't guess what Category or Sub-Category the user will choose to classify files into (though it does pick everything else out of the ID3 tag🙂.
-geof