Superceded. See the Optimal OPML Browser.
I've tweaked it quite a bit. For example, I think it can handle just about any OPML you throw at it, no matter how far out of spec. However, it's still very much a work in progress.
The HTML rendering is performed almost entirely with XSL transforms (with the exception of (1) URI encoding the query string passed back to the browser to "expand" included OPML links and (2) mangling the author's email address a little bit). Update: It's all XSLT, now.