Just to sketch how I would try to solve this: Write an
extension to the
appropriate hook (see
http://www.mediawiki.org/wiki/Manual:MediaWiki_hooks). When a page is
requested, look up if a page with that title (case insensitive) exists.
If yes, perform an external (HTTP) or internal redirect (a matter of
taste, I guess) to the page with the requested title in its canonical
form. If no, proceed as usual.
I've just written a patch that creates a new setting similar to
$wgCapitalLinks, but which puts the whole title to uppercase, rather
than just the first letter. You could then use {{DISPLAYTITLE}} on
each page to determine what case actually gets displayed. Would people
be interested in that (once I've ironed out the bug that completely
breaks all special pages, anyway...)?