Users are occasionally surprised to find that certain character sequences in their text have been unexpectedly replaced by smilies, thumbs, or other surprise graphics. For example, see http://moodle.org/mod/forum/discuss.php?d=63913.
Why not move the processing of text-based smilies into an optional text filter? Then users can turn them on or off at site level. Users will still be able to insert smilies into their text using the HTML editor toolbar icon.