What is google.com/aclk ?

The Earth Hour link on the Google search page for March 28, 2009 links to:

www.google.com/aclk
?sa=L
&ai= [a long code]
&num=1
&sig= [a signature]
&q=XXttp://www.google.com/intl/en/earthhour/2009/
[Note: I substituted "XX" for "h" so the forum would not linkify it.]

Earth Hour: http://blogoscoped.com/forum/152466.html

The search for [inurl:google inurl:aclk] http://www.google.com/search?q=inurl%3Agoogle+inurl%3Aaclk&num=100 produces 34 results.

One has a reference to "google ad services page aclk": http://www.uslanmam.com/konular/192780-google-ad-services-page-aclk
It also seemed to reference google news wires services.

Here are some occurences in blogoscoped.

http://blogoscoped.com/search/?q=aclk
http://www.google.com/search?&q=site%3Ablogoscoped.com+aclk&num=100

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 (http://www.googlebooksettlement.com), pass your mouse over the ad, the URL is also : http://googleads.g.doubleclick.net/aclk?sa=l ...

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.