This is a cross-post to wikipedia-l and wikitech-l.
I just switched the version of MediaWiki we're running on Wikimedia websites to CVS HEAD. Currently it's labelled 1.6alpha, but development is continuous.
What this means for users is that features will appear on the site as soon as possible after they are written, instead of waiting for the next major release of MediaWiki in up to 6 months' time.
There were very few user-visible changes associated with this switch, the main difference will be development practices in the future. One user-visible change you might notice is that if a page is deleted while you're editing it, you'll get a kind of edit conflict.
What this means for developers is that we'll have to be more careful what we put into HEAD. Test everything thoroughly, or put experimental features into their own branch. You should backport bug fixes to the stable branch (currently REL1_5), but it's no longer necessary to backport features.
With a more stable CVS HEAD, we should be able to release major versions of MediaWiki more often.
This development schedule was suggested by Brion and enthusiastically seconded by me.
-- Tim Starling