Genesis 1.8.x Post Class Patch for functions.php. This patch will be safe for Genesis 1.9.x re-write of this function.

inpost-metaboxes-save-patch.php

<?php

add_action( 'save_post', 'wps_inpost_layout_check', 1 );
/**
 * Checks the version of Genesis and executes accordingly.
 *
 * Executes for Genesis versions less than 1.9.x.
 *
 * @category Genesis
 * @package Admin
 * @subpackage Inpost-Metaboxes
 */
function wps_inpost_layout_check() {
	if ( version_compare( wp_get_theme()->parent()->Version, '1.9.0' ) < 0 ) {
		add_action( 'save_post', 'genesis_inpost_layout_save_patch', 2, 2 );
	}
}

/**
 * Saves the post class when we save a post / page.
 *
 * It does so by grabbing the array passed in $_POST.
 *
 * @category Genesis
 * @package Admin
 * @subpackage Inpost-Metaboxes
 *
 *
 * @param integer $post_id Post ID
 * @param stdClass $post Post object
 * @return integer|null Returns post ID if the nonce was not correct or user was not
 * allowed to edit content. Returns null if doing autosave, ajax or cron
 */
function genesis_inpost_layout_save_patch( $post_id, $post ) {

	/**	Don't try to save the data under autosave, ajax, or future post. */
	if ( defined( 'DOING_AUTOSAVE' ) && DOING_AUTOSAVE )
		return;
	if ( defined( 'DOING_AJAX' ) && DOING_AJAX )
		return;
	if ( defined( 'DOING_CRON' ) && DOING_CRON )
		return;

	$genesis_custom_post_class = $_POST['_genesis_custom_post_class'];

	/** Save custom post class value, or delete if the value is empty */
	if ( $genesis_custom_post_class )
	    update_post_meta( $post_id, '_genesis_custom_post_class', $genesis_custom_post_class );
	else
	    delete_post_meta( $post_id, '_genesis_custom_post_class' );  


}