source: trunk/www.guidonia.net/wp/wp-content/plugins/contact-form-7/modules/submit.php@ 44

<?php
/**
** A base module for [submit]
**/

/* Shortcode handler */

function wpcf7_submit_shortcode_handler( $tag ) {
        if ( ! is_array( $tag ) )
                return '';

        $options = (array) $tag['options'];
        $values = (array) $tag['values'];

        $atts = '';
        $id_att = '';
        $class_att = '';

        foreach ( $options as $option ) {
                if ( preg_match( '%^id:([-0-9a-zA-Z_]+)$%', $option, $matches ) ) {
                        $id_att = $matches[1];

                } elseif ( preg_match( '%^class:([-0-9a-zA-Z_]+)$%', $option, $matches ) ) {
                        $class_att .= ' ' . $matches[1];

                }
        }

        if ( $id_att )
                $atts .= ' id="' . trim( $id_att ) . '"';

        if ( $class_att )
                $atts .= ' class="' . trim( $class_att ) . '"';

        $value = $values[0];
        if ( empty( $value ) )
                $value = __( 'Send', 'wpcf7' );

        $ajax_loader_image_url = wpcf7_plugin_url( 'images/ajax-loader.gif' );

        $html = '<input type="submit" value="' . esc_attr( $value ) . '"' . $atts . ' />';
        $html .= ' <img class="ajax-loader" style="visibility: hidden;" alt="ajax loader" src="' . $ajax_loader_image_url . '" />';

        return $html;
}

wpcf7_add_shortcode( 'submit', 'wpcf7_submit_shortcode_handler' );

?>