Update: I just encountered exactly the same problem again. For JSP and Servlets it works fine (although the JSP one should apply XML/HTML syntax highlighting rather than Java syntax highlighting, but that aside), but for Servlet Filters it doesn't work at all, while it does work in edit page. Does syntax highlighting need to be configured on a per-tag basis or so?
Update 2: the highlighter also doesn't run in JSTL info page where I'd expect XML-like syntax highlighting. After investigating the CSS it turns out that SO is using a tag-specific classname (which matches the most popular tag) to trigger language-specific syntax highlighting. I'd suggest to add a default classname which triggers automatic highlighting for the case that there's no match in tags.
That said, I'm also not very happy to see that JSP (and JSF) posts uses Java-targeted syntax highlighting. It should be using XML-targeted syntax highlighting (or better, automatic, since it's often a mix of Java code and XML-style code in those posts). However, most JSP questions are posted along with a
[java] tag as well which causes the wrong syntax highlighter to be triggered. About half a year ago everything in JSP/JSF/JSTL tags was highlighted perfectly.
Or does that deserve another question?