Google says:

Today we’re launching SearchWiki, a way for you to customize search by re-ranking, deleting, adding, and commenting on search results. With just a single click you can move the results you like to the top or add a new site. You can also write notes attached to a particular site and remove results that you don’t feel belong. These modifications will be shown to you every time you do the same search in the future. SearchWiki is available to signed-in Google users. We store your changes in your Google Account. If you are wondering if you are signed in, you can always check by noting if your username appears in the upper right-hand side of the page.

The changes you make only affect your own searches. But SearchWiki also is a great way to share your insights with other searchers. You can see how the community has collectively edited the search results by clicking on the “See all notes for this SearchWiki” link.

This could be pretty big.  Or a pretty big headache, once people start spamming it.  It will be interesting to see how this rolls.