Google removed itself from the IAB's working group on safe frame, which fought for user and publisher safety. among things like preventing the ad from knowing what's on the page it's served on it also handle if the ad is in view, preventing most of those bot net traffic that uses 1x1 iframes or insert ads with low zindex on the page. the only vulnerabilities left with safeframes are giving the user a executable when they click on the ads (nothing can prevent that) and flash exploits (although the most common exploits are blocked fine, it's still vulnerable when it's a full control bug in flash)

but Google got out when they realized it wouldn't give as much info from its adsense network of third party sites as they are used to with insecure adwords (when it shows display ads)