MediaWiki 1.5.6 and 1.4.14 are security and bugfix maintenance releases.
A bug in edit comment formatting could send PHP into an infinite loop if certain malformed links were included. In most installations, this would cause the script to fail after PHP's 30-second failsafe timeout.
Release notes:
1.5.6: http://sourceforge.net/project/shownotes.php?release_id=386609
1.4.14: http://sourceforge.net/project/shownotes.php?release_id=386608
Download:
http://prdownloads.sourceforge.net/wikipedia/mediawiki-1.5.6.tar.gz?download
http://prdownloads.sourceforge.net/wikipedia/mediawiki-1.4.14.tar.gz?download