Matt announced today at the 2011 State of the Word that plugins and themes are now hidden from the search results (on both WordPress.org and in the plugin/theme install screens) if they have not been updated in two years.

For plugins only: Older plugins that are still compatible and secure only need to have their “requires” and “tested up to” version numbers bumped. If you do this, actually releasing a new version of the plugin is not necessary, so leave the version number the same.