f your page is http then it allows iframe with https content.
But if your page is https then it does not allow http content.
Lets put down following possibilities.
page - iframe - status
http - http - allowed
http - https - allowed
https- http - not allowed
https- https - allowed ? (Need to confirm)
stackoverflow : http://stackoverflow.com/questions/9280665/insecure-content-in-iframe-on-secure-page
