Here is what the new policy says :
Cookies & Web beacons:
Most adsense publishers are familiar with http cookies (or web cookies). They are a small parcels of text sent by a server to a web client(browser) and then sent back unchanged by the client each time he/she accesses that server. But the term Web beacon may be new to you.
A web beacon (also known as tracking pixel or pixel tag or clear GIF) is a small transparent graphic image (usually 1 pixel height and 1 pixel width) that is placed on a website or even in an email that is used to moniter the behavior of the client/user visiting the webpage, to find out some user information like IP address,etc.