Manipulating iframe content

Hello,

I face some problem, i am using jquery as my framework, and i got an issue with the iframe content.

What i want to do is the load external website into my iframe (random website url from my database), and to insert my script into header (i want to do something like greasemonkey does on firefox).

Here's my code:

$('iframe').contents().find('header').append('<script>My user script</script>');

The problem is that this doesn't work, when i tried to do $('iframe').contents().find('header').html() returns nothing...

I knew this was a limitation in html because of cross domain, but adobe air isn't restricted to that, so what am i doing wrong here?