"A major update that Google is testing has completely broken the ability for any external analytics service like Clicky to determine the search query used by a visitor arriving at your web site. (...) If this update goes live for everyone, it effectively means that 2/3 of all searches leading to the average web site will be a complete mystery," explained Clicky's blog.
Here's Google's explanation from March: the Ajax-enhanced pages were tested on a small number of users and this change could make Google's SERPS faster and smoother.
To solve the problem, Google will use a gateway URL for all search results.
Starting this week, you may start seeing a new referring URL format for visitors coming from Google search result pages. Up to now, the usual referrer for clicks on search results for the term "flowers", for example, would be something like this:
http://www.google.com/search?hl=en&q=flowers&btnG=Google+Search
Now you will start seeing some referrer strings that look like this:
http://www.google.com/url?sa=t&source=web&ct=res&cd=7&url=http%3A%2F%2Fwww.example.com%2Fmypage.htm&ei=0SjdSa-1N5O8M_qW8dQN&rct=j&q=flowers&usg=AFQjCNHJXSUh7Vw7oubPaO3tZOzz-F-u_w&sig2=X8uCFh6IoPtnwmvGMULQfw
Niall Kennedy suggests that "Google is likely making this change to better track search actions and shield URL parameters from sites downstream", but I think this is a solution for the lack of referral information in a future Ajax interface.
Google is already using gateway URLs for users that are logged in: just copy the address of a Google search result and you'll notice that it's just a redirect which sends Google some details about your actions.