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.
This happened before when Google switched to testing AJAX based results, which caused an uproar amongst webmasters who were unable to track referrers.
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.
