Having read i18n_characters_in_post_title, I have a page named St John's in a file named St_John__39__s.mdwn. Regular wikilinks like \[[St_John's]] successfully point to that page. However, if I tag a page with [[!tag St_John's]], that link is shown as pointing to a non-existant page. Modify the tag to read [[!tag St_John__39__s]] works around the problem.