Matt Curtis wrote:
In w:Help:Link it states clearly that "in internal and interwiki link style, a plus sign in a page name is not allowed, the HTML and hence the rendered page just shows the wikitext, e.g. a+b".
In MediaWiki 1.6, you can optionally add to the set of allowed title characters.
This breaks backwards compatibility with some very old Wikipedia URLs from 2002, so is somewhat controversial. (eg if you follow one of those a very old links that has been recorded, linked, or referenced somewhere it no longer brings you to the right page.)
-- brion vibber (brion @ pobox.com)