It looks like Google is marking all of its search results with this warning: "This site may harm your computer."
If you click on a Google result link in spite of the warning, you get an interstitial page with an additional warning: "Warning - visiting this web site may harm your computer!"
Clicking the warning itself will take you to this page, which explains: "This warning message appears with search results we've identified as sites that may install malicious software on your computer."
The server(s) that hosts that page seems to be getting hammered right now. No surprise. So is StopBadware.org, a site Google refers its users to for more information.
And so is the Google server that provides more detailed diagnostics for sites allegedly failing Google's safety tests, such as this report for the presumably clean Wikipedia site.
Coincidentally, I was reviewing the diagnostics page just yesterday for a site that had been infected by malware. The diagnostics page identified the origin of the malware that Google spotted on the server it was warning about. I wonder what, if anything, Google is saying about all these allegedly infected sites this morning... especially since Google is warning about its own site as well, if your search encompasses one of Google's own pages.
Something, I think, is amiss in Mountain View...
UPDATE: Todd Gardner suggests this problem was caused by an outage at StopBadware.org. If so, Google needs to rethink its fail-safe strategy for this data source.
UPDATE 2: Google seems to be working normally again.
UPDATE 3: According to a comment on this post by CNET user stopbadware, manager of StopBadware.org, and a blog post on that site titled Google glitch causes confusion, the problem was not with StopBadware.org.
UPDATE 4: According to this post on the official Google blog site by Marissa Mayer, Google vice president of search products & user experience, the problem was caused by a bad update to Google's list of malware-infected Web sites. As the post says, "the URL of '/' was mistakenly checked in as a value to the file and '/' expands to all URLs."
Mayer reports that the problem has been fully solved and promises that Google "will carefully investigate this incident and put more robust file checks in place to prevent it from happening again."