A new WebmasterWorld thread reports several webmasters complaining that Google is not sending referrer information for all clicks from a Google search engine.
First reports came from WebmasterWorld yesterday, July 12th. The individual said:
More and more visits from Google in my server log files are without exact referrer information, and have only "http://www.google.com", "http://www.google.com.au", etc. which doesn't allow to find out keyword and SERP page from which this visit was made.
Several other webmasters confirmed seeing the same issue. Zett, a senior member said, "I am seeing that as well. Not too many yet (maybe 2 to 3% of all referrers from Google), but still noticeable."
Another person posted what he thinks it might be:
I can confirm the original observation: more and more accesses just contain a bare "http://www.google.foo/" referrer. It started on July 1st/2nd (a rise from about .1 percent to about 3 percent of all Google referrers), and judging by the HTTP headers and access patterns, these are definitely legit accesses, not bots, in particular as a possible rel=prefetch access (for top-ranked search hits under Mozilla-based browsers) directly preceding the actual user-driven access still _does_ contain the full list of Google search parameters in the referrer.
There was some discussion that Google would being back the AJAX results after they figure out a way to pass along the referrer data. Not sure if this is them testing this or something else.
Forum discussion at WebmasterWorld.
Update: Matt Cutts of Google confirmed an issue when Google was not sending referrer data in the thread. He said:
Hey everybody, I asked folks who would know about this. It turns out there was an issue a couple weeks ago where some code got refactored, and the refactoring affected referrers for links opened in a new tab or window. Right now the team is expecting to have a fix out in the next week or so. Hope that helps.