I was unable to find anything that made it easy to paste iframe embed codes into the visual editor. The existing solutions all required me to transpose info from the embed code to the shortcode. This allows you to simply paste your iframe code. Just put it inside [iframe][/iframe].

Update: Now available in repository as plugin ready to go

http://wordpress.org/extend/plugins/iframe-shortcode/

add_shortcode( 'iframe', 'simple_iframe_sc' );

function simple_iframe_sc( $atts, $content = null ) {
    $char = array( '‘', '’', '“', '”', '′', '″' );
    $replace = array( "'", "'", '"', '"', "'", '"' );
    return html_entity_decode( str_replace( $char, $replace, $content ) );
}

Example use:


[iframe]<iframe width="560" height="315" src="http://www.youtube.com/embed/KuxWu1D3Mk4" 
frameborder="0" allowfullscreen></iframe>[/iframe]