function GenerateIframe( $atts ) {
    extract( shortcode_atts( array(
        'href' => 'http://the-url',
        'height' => '550px',
        'width' => '600px',     
    ), $atts ) );

    return '<iframe src="'.$href.'" width="'.$width.'" height="'.$height.'"> <p>Your Browser does not support Iframes.</p></iframe>';
}
add_shortcode('iframe', 'GenerateIframe');
// how to use: [iframe href="http://www.exmaple.com" height="480" width="640"]