Summary says it all; dev-lang/php-5.3.2 has an unstated 'mbstring' use flag in use, but it's not in it's IUSE (naughty naughty), nor metadata.xml, nor use.desc.
Yes. That's fixed now, thanks. Still needed to confirm whether mbstring supports building with external oniguruma now. I appreciate bugs, but please do not edit the ebuild yourself. If you do, please add a Changelog entry, as I normally don't follow -commits.