For years Web publishers using Adsense have been unable to display advertisements on secure pages (HTTPS).
That's no longer the case as Google this week announced that they have updated the Adsense ad code so that it now supports secure ad serving through Secure Sockets Layer (SSL) on HTTPS web pages.
Essentially, publishers with secure sites, mostly ecommerce and social networks sites, or any site really that protects the sensitive information (credit card, login credentials) of its users can use Adsense to serve SSL-compliant ads.
In the past if a publisher tried to display Adsense on secure pages, their users would be shown what is known as a mixed content warning. That was less than optimal of course, so this improvement will likely be well received.
Below are some examples of "mixed content warnings"