The cookie is in doubleclick.net domain, by the name id. It has a unix timestamp in it, which should show you how old your cookie is. They expire it after 1 year.
This is a good initiative on the part of Google, but here is the bit that's scary.
My cookie expires Jan 16, 2012, which means its barely a month old. In this period, they have been able to figure out that I am interested in Advertising and Marketing, Computer & Electronics->Programming, Software & Computer Security. Of course - their score is not 100%, but based on what I did during last month, thinking that I am interested in movies (I purchased a ticket for Avatar from AMC Entertainment) and Watches (I bought a few), they are close.
Too close for comfort. Would I opt out? No. But the average user might.