The links have to be the kind that do not pass
PR (by not being crawled by robots) - so they have to be opened through either:
1) html links with rel="nofollow" in the <a href ..> tag
2) javascript pop-up windows (like Google Adsense)
3) a redirector page wich uses a robots meta tag with "noindex,nofollow" or is disallowed in robots.txt, and uses meta refresh to the external link
Google Adsense are javascript, do not pass
PR, they don't get crawled by robots.