If you're taken from one https to another https location, doesn't referrer still get passed? So possibly what it means is that you need to implement SSL. Google's choice here will hopefully motivate change around the web.
It depends. For Gecko, that's a configuration option (defaulting to "send cross-site" at the moment, but subject to change in general).
And as others have pointed out, sites using AdSense can't very well "implement SSL".
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