If you haven't heard about this already you are automatically opted in by Google to this new feature. if you want to disable it you can in your adsense control panel. I don't like it myself as it may weaken some of my own advertising that I do privately on my sites. It would have been nice if Google gave you the option to opt in first rather than just opting everyone in by default.

There is an interesting thread about it at:

http://www.webmasterworld.com/forum89/10563-1-10.htm

Cheers,

stevey