On the Inside AdWords blog, the Google AdWords team says that the AdWords Site Stats logo is no longer mandatory. The post says that there was a "Google Site Stats" text area that would be displayed when a conversion occurred which gave visitors information about AdWords conversion tracking (and instructions on how to remove it if you never wanted it).
Given a variety of negative feedback, like this WebmasterWorld thread, for example, Google has now given you the opportunity to remove it.
If you want to remove it, you will have to actually opt out manually. This is not an automatic process. Instructions for doing so are here.
Forum discussion continues at WebmasterWorld.