In a recent interview with Cedric Dupont, product manager for Google SearchWiki, we found that Google doesn't intend to provide an option to disable SearchWiki. "While users don't have the option of turning off SearchWiki, they do have the option not to use the feature. By turning off the feature entirely, people will never get used to the new offering or see how it might be useful to them. We encourage people to try it out."
Fortunately, there are some ways to hide SearchWiki's visual clutter:
1. Sign out from your Google account. The feature is only available to authenticated users since it's a way to customize search results.
2. Use a Greasemonkey script that hides the icons added by SearchWiki.
3. To disable SearchWiki for the current session, add "&hl=all" to the URL of a search results page. Unfortunately, the change is not permanent. Example: http://www.google.com/search?q=test&hl=all.
4. If you enable any Google Experimental feature except "SearchWiki with sound", you'll no longer see Google SearchWiki. Probably the most useful experiment you can enable is "Keyboard shortcuts", which adds shortcuts to Google search. Joining an experiment changes your Google cookie, so the experiment is enabled in the current browser and only until you delete Google's cookie.
SearchWiki aficionados can enable "SearchWiki with sound" to hear a sound effect when a search result is removed. "Do you enjoy having the power to remove results from your search results pages? Now you can do so in style by having a sound effect play along with the animation whenever you remove a result. Recorded by our co-founder Sergey Brin himself, this sound effect is sure to please!"
Update: You can now disable SearchWiki from Google's preferences page.