To get the window object for a frame you can use the window.frames array:

var iframewindow= frames['iframe_name'];

This requires that you give the <iframe> an old-school name attribute instead-of-or-as-well-as the id. Alternatively if you know the order of iframes on the page you can index them numerically:

var iframewindow= frames[0];

It's generally more flexible to get the iframe window from the iframe element in the DOM, but this requires some compatibility code to cope with IE:

var iframe= document.getElementById('iframe_id');

var iframewindow= iframe.contentWindow? iframe.contentWindow : iframe.contentDocument.defaultView;

jQuery defines the contents() method to grab the document node, but it doesn't give you a cross-browser way to go from the document to the window, so you're still stuck with:

var iframe= $('#iframe_id')[0];

var iframewindow= iframe.contentWindow? iframe.contentWindow : iframe.contentDocument.defaultView;

which isn't really a big win.

(Note: be very careful using jQuery for cross-frame-scripting. Each frame needs its own copy of jQuery and methods from one frame's copy won't necessarily work on nodes from the other. Cross-frame-scripting is a topic fraught with traps.)