If you do something like this...

var myFrame = document.getElementById('myIFrame');

var framedDoc = myFrame.contentWindow.document;

var theOffset = $('#someElement', framedDoc).offset();

The value returned is incorrect as it is the offset of the element relative to the contentWindow of the iframe, not the window where jQuery is running. To corrent this, the offset of the iframe element in the parent window must also be added to the offset.

Attached is a patch fixing this issue that I've tested in a few browsers. Just needs a more exhaustive test and commit.