A syntax highlighting extension based on GeSHi is now live on test.wikipedia.org. There's a test page at:
http://test.wikipedia.org/wiki/SyntaxHighlight_GeSHi
The documentation is now out-of-date, but it might contain some useful information:
http://www.mediawiki.org/wiki/Extension:SyntaxHighlight_GeSHi
Some comments and background about the review process are at:
http://bugzilla.wikimedia.org/show_bug.cgi?id=7163
I'd like to invite comments now. I'll be aiming to deploy it on the main site some time in the next week.
-- Tim Starling