Recently our main spam URL blacklist at http://meta.wikimedia.org/wiki/Spam_blacklist has gotten too long for PHP's regulation expressions library (PCRE) to allow. As a result, the check would more-or-less silently fail.
I've updated the spam blacklist extension to split a long list into multiple regexes, so it will handle the larger list without exploding.
Third-party MediaWiki sites using our default blacklist should consider updating it from SVN if you've noticed the blacklist has stopped working for you.
The anonymous SVN checkout URL is still: http://svn.wikimedia.org/svnroot/mediawiki/trunk/extensions/SpamBlacklist
You can also poke at the files in a human-readable fashion: http://svn.wikimedia.org/viewvc/mediawiki/trunk/extensions/SpamBlacklist/
The cleanup.php script hasn't been tested yet for the update, so if you use it, well, say something if it's borked. :)
-- brion vibber (brion @ pobox.com)