I'm new to the Mediawiki codebase, but after going to Wikimania, I've been impressed with the need for more eyes on Code Review.
Since I'm still learning my way around, I won't be able to approve any code (e.g. mark it “ok”) but I can offer feedback where it might be helpful.
Right now, I'm focusing on Core, LiquidThreads and the Usability Initiative extensions. If you have an extension that you'd like feedback on, let me know and I'll have a look.
I do not pretend to have Tim's expertise, so this is not a substitute for his review, but it could be a way to get some preliminary feedback as well as helping me to become more familiar with various parts of MediaWiki and its components and extensions.
Your friendly code gnome,
Mark.