Google is adding the "Ads by Google" notice to rich media ads, such as Flash and interactive ads. It will appear as an information icon (lowercase "i" in a small circle) in the lower right corner. When the icon is hovered over, it expands to the words "Ads by Google." If the notice is clicked on, then the user is taken to a page about Google's advertising practices.
Considering news of this update was published not only on the AdWords and AdSense blogs but also on the Google Public Policy blog, it's safe to assume that this is a reaction to political scrutiny. Politicians from state and federal governments have been looking into online behavioral advertising practices, wary of the implications it has for privacy.