Responsive iframes, requires jQuery

responsive-iframes.js

// Find all iframes
var iframes = jQuery("iframe");

// Find & save the aspect ratio for all iframes
iframes.each(function () {
	jQuery(this).data("ratio", this.height / this.width)

		// Remove the hardcoded width & height attributes
		.removeAttr("width")
		.removeAttr("height");

});

// Resize the iframes when the window is resized
jQuery(window).resize(function () {
	iframes.each(function() {

		// Get the parent container's width
		var width = jQuery(this).parent().width();
		jQuery(this).width(width)
			.height(width * jQuery(this).data("ratio"));

	});

});

// Resize to fix all iframes on page load.
jQuery(document).ready(function() {
	jQuery(window).resize();
});

Props to Ben Marshall for the JS.