Third-party ads won't include the 'Ads by Google' text, and will open in a new browser window. However, because all third-party ads must abide by Google's image ad and content policies, you likely won't notice any differences at all.

No "ads by Google" and a new window? - I think that's a huge difference. (I like the new window part!)

It also seems that anyone with image ads enabled is automatically a part of this. Or am I reading this wrong?