AdSense is available as https
That is annoying. Sure would be nice if the ad networks had a quick FAQ to find out if they offer their feed on SSL, before you sign up.
OTOH, if an ad network offers their feed as a raw XML web service (or something like that), you can write a proxy script that requests the ads, then serves them from your own domain on SSL. YPN used to work that way, as did Overture.
But if the ad network merely works via a drop-in <script> (many do) and the @src isn't available on SSL, then that won't work. :(