It would let Firefox and IE7 autodiscover Confluence as a search engine.

That is, navigating to a Confluence 2.10 and above instance should now present users the opportunity to add that site onto their list of search engines. They can now search that confluence site from the convenience of their browser search box anytime.

This feature does not include presenting Confluence search results in XML or in a format consumable by OpenSearch aggregators/syndicaters.

A corresponding feature request for JIRA is in JRA-12309.