In my previous post about the new dialog in IE8 about mixed content, someone asked "How can I prevent this message from reappearing?", I thought it would be a good-to-have post and started writing this, but someone else already answered it (thanks Anonymous), in any case, here's what you need to do if you don't want to see that dialog:
tools>internet options>security>custom level>display mixed content: enable
Note that if the site you are visiting is NOT on the internet zone, you would have to make the changes to the appropiate zone:
here's how you tell which "zone" you are on, on the bottom right hand corner of the browser you should see something like this:
If you double click that, you can make changes to the other zones (Local Intranet, Trusted sites, restricted sites)
Just click the zone you want, and it will bring up the first dialog (from this post) where you can make the change.
This applies to IE8, IE7, IE6 (I don't know about older versions)