Wordpress - Make WordPress WYSIWYG not strip out iframe's

You can customize the filter of TinyMCE, see the following example for <iframe>s and other tags to use Google Maps inside TinyMCE.

function fb_change_mce_options( $initArray ) {

    // Comma separated string od extendes tags.
    // Command separated string of extended elements.
    $ext = 'pre[id|name|class|style],iframe[align|longdesc|name|width|height|frameborder|scrolling|marginheight|marginwidth|src]';

    if ( isset( $initArray['extended_valid_elements'] ) ) {
        $ext = ',' . $ext;
    }
    $initArray['extended_valid_elements'] = $ext;

    // Maybe, set tiny parameter verify_html
    //$initArray['verify_html'] = false;

    return $initArray;
}
add_filter( 'tiny_mce_before_init', 'fb_change_mce_options' );

Add this to an custom plugin or functions.php of the theme. Also you can read more informations in my post: http://wpengineer.com/1963/customize-wordpress-wysiwyg-editor/

If you don't want to write your own code there is a plugin to allow embedding an <iframe>:

• Embed IFrame Plugin for WordPress

Then just use the shortcode like this:

[iframe http://example.com 400 500]