We've changed the behavior of mojoPortal to try to keep you in ssl once you have started an ssl session. This is discussed in the article Use SSL.
Basically once you are signed in we want to keep you in ssl to protect your authentication and role cookies from being stolen which would allow another person to access your site with your privileges. We do this by using relative urls as much as posisble since these will inherit the protocol from the current page so once you get into ssl it generally stays in ssl unless you click a link that explicitly uses http.
I've encountered this same problem on this site with adsense. If you only configure adsense for text based ads, it works fine on secure pages but if you allow rich media or images it can result in browser warnings.
The only real solution is for google to fix adsense so it fully supports ssl for ads. It is actually kind of mind blowing that they haven't done this since they led the charge to use ssl everywhere in their other services.
You "could" go back to the old behavior by adding this to user.config:
<add key="ClearSslOnNonSecurePages" value="true" />
but then the downside is that you will either put your cookies at risk or you will appear to be not logged in on insecure pages if you use the recommended settings to protect the authentication cookie as discussed in the linked article. You will notice on this site that if you are signed in but somehow get to an url with only http and not https then you don't appear to be logged in because I have it configured to protect the cookies by only passing them on secure requests.
Hope that helps,