Wikimedia sites will gradually be upgraded to version 1.20wmf1 of MediaWiki in April 2012.
The latest version (labeled "1.20wmf1") of MediaWiki, the software that powers Wikipedia and its sister sites, is currently being deployed to all Wikimedia sites in stages.
We started on April 10th and will continue until April 25th (see the roadmap).
From now on, we plan to deploy the latest software every two weeks. This is intended to minimize disruption and possible issues, and make it easier to identify the cause of problems, since the possibly problematic code will be much more recent.
If, despite our efforts, you encounter issues due to the upgrade; we'll try and fix them as soon as we can. Please report issues in our bug tracker, which is where we look for reports of problems. And the faster you tell us about problems, the faster we can address them.