That was weird: searching for anything on Google was returning all results marked as "This site will harm your computer". Even searching for Google:

I submitted a report to Google, and in a few minutes Google.com was corrected. Google.co.uk showed bad results for one more search, but now that too is corrected. I couldn't find any mentions elsewhere of this yet, but it occurred both from home and via my work PC.

E: looks like the culprit is StopBadware.org: they seem to provide this information for Google, and their site is currently down.

E2: StopBadware.org put the blame back on Google in their blog entry: they say their site went down because of millions of people clicking through to it from the warnings, which were falsely generated because of a glitch at Google's end.