Most people have probably forgotten about it by now, but back at the start of march[1] there was a discussion about a partial rewrite of the Title system (though I use rewrite to heavily when I'm talking about a big backend project, even when 90% of the system stays the same). It was branched off the "[Wikitech-l] Case insensitive links (not just titles)." thread.
The branch failed twice back when I was working on it in March. Primarily I ran into the twilight zone known as branching and merging using SVN on Windows... eugh, and the branch always got messed up and was unmaintained.
Now, I've got my own laptop running Ubuntu, and branching has gotten a fair bit easier.
I've restarted the titlerewrite branch again. Anyone interested can check it out and discuss the project as well. Just to recap for those who didn't see the old discussions, as I remember there were 3 goals: A) Make MediaWiki understand the concept of titles like "iPod" By "understand", I mean [[IPod]] and [[iPod]] are still the same page, and they still show up as IPod in the url and database. However, MediaWiki also has "iPod" stored so it knows what the actual styled format of the title is. Basically this is handled by adding a column into the database. The difference between this and {{DISPLAYTITLE:...}} is that displaytitle is a parser hack, while this is an actual integrated part of the Title backend. If you rename IPod to iPod on Wikipedia with this rewrite, then Special:Allpages and everywhere else, will display "iPod" instead of IPod like it currently does. B) Replace Title::secureAndSplit with an extensible Normalizer. There are a lot of requests in bugzilla and the lists here for features like full case-insensitive titles, per-namspace case sensitivity, changing MW's _ handling to use - instead, and so on... Some of these can be handled fine with adding new configuration into MediaWiki. However, there is a point where it's unreasonable to add new configurations for minor changes to titles that people might want. The idea behind the Extensible Normalizer would be that extensions or other things would be able to modify the normalization process for titles. C) Repurpose and migrate the DISPLAYTITLE hack into it's own system. Currently the DISPLAYTITLE parser function is merely used to change things like IPod into iPod. While the introduction of A) would make the use od DISPLAYTITLE no longer necessary, people do make use of DISPLAYTITLE with restrictions turned off, or the old {{Title}} hack to further alter the title. Sometimes a wiki has a valid reason for wanting things like italics or superscript inside a small part of the title. Repurposing DISPLAYTITLE would mean changing it from something meant to tweak page title, and the html header, to just the html header with different levels of restriction. In extension to this, the plan was to move displaytitle partially out of it's limited area inside of the parser cache, and make it a separate extensible system. While it is nice to allow people to do things like changing "Lisp (programming language)" to "LISP" when restrictions are lowered, and using things like "NR-N99 /Persuader/-class droid enforcer" (note the italics) it's even better if we can give Extensons the ability to tweak the display title that way. Think of cases like a MiniWiki extension running on a wiki that runs multiple small wiki in it, like Wikia's Scratchpad wiki. It would be possible for the extension to modify the title, and perhaps even link in the root of the mini wiki.
For anyone who's taken a look at the old branch, there is one thing to note. In the old schema the plan was to use a page_title_ui column, but this time around I'm going for a rev_title_ui column. This, among other things will allow versioning of that title, but will also make deletion and undeletion a little cleaner.
As for the current branch, here's a screenshot or two: http://d.imagehost.org/view/0511/Screenshottitle_Development_Environment_tit... http://d.imagehost.org/view/0768/Screenshot-All_pages_Development_Environmen...
You can notice the title in the first, that is not done with Displaytitle. And in the latter you can see the difference between [[Title]] which has a rev_title_ui of "_title_" and [[Displaytitle]] which uses {{DISPLAYTITLE:_displaytitle_}}.
I didn't pull up any more recent screenshots, but User: pages are now affecting how a user's username is displayed around the site (including history and rc pages after a little tweak, and in the skin as well), and viewing old revisions of pages respects what the ui title was during that revision.
[1] http://lists.wikimedia.org/pipermail/wikitech-l/2008-March/thread.html