Thread:Project:Support desk/Default Value "Search" in Search Box (Vector)/reply (3)

This enhancement, among other nice features, is part of the Extension written for the Vector-skin (Extension:Vector). It is written recently and requires MediaWiki 1.17. Although 1.17 isn't officially stable yet, Wikipedia is currently running it on all wikis.

If you decide to upgrade to 1.17, be sure to keep up with updates and make sure you make backups before every update. Upgrading to 1.17 is not yet recommended for large/production sites.