What is google.com/aclk ?
The Earth Hour link on the Google search page for March 28, 2009 links to:
&ai= [a long code]
&sig= [a signature]
[Note: I substituted "XX" for "h" so the forum would not linkify it.]
Earth Hour: blogoscoped.com/forum/152466.h ...
The search for [inurl:google inurl:aclk] google.com/search?q=inurl%3Ago ... produces 34 results.
One has a reference to "google ad services page aclk": uslanmam.com/konular/192780-go ...
It also seemed to reference google news wires services.
Here are some occurences in blogoscoped.
Apparently it was added to robots.txt around September 2007.
I'm pretty sure this is just a redirect for tracking click-throughs.
There is currently an ad for Google Books on several websites (googlebooksettlement.com), pass your mouse over the ad, the URL is also : googleads.g.doubleclick.net/ac ... ...
BTW, some ads for this website are tracking exactly the position of the mouse when you click on it!
If you examine a SERP, "aclk" is used for the ads. The SERP uses a different tracking for the organic results.
The Earth Hour link on the search page used the ad tracking, but it did not look like an ad.
FWIW there's currently a link on Google.co.uk advertising Google Street View which links to Google Maps using an "aclk" URL.