change the class of a post based on a postmeta value in WordPress

post-layout.php

<?php
/*
Plugin Name: wpse32973 Post Layout
Author: Christopher Davis
Author URI: http://www.christopherguitar.net/wordpress/
Description: Change the post class (for styling purposes) so posts can be formatted correctly.
*/

// register the meta box
add_action( 'add_meta_boxes', 'wpse32973_add_meta_box' );
function wpse32973_add_meta_box()
{
	add_meta_box(
		'wpse32973-box',
		__( 'Post Layout' ),
		'wpse32973_meta_box_cb',
		'post',
		'side',
		'high'
	);
}

//
function wpse32973_meta_box_cb( $post )
{
	$meta = get_post_meta( $post->ID, '_wpse32973_layout', true );
	
	// what options you'd like to have for layouts.  value => label
	$opts = array( 
		'right_thumb' => __( 'Right Thumbnail' ),
		'left_thumb'  => __( 'Left Thumbnail' )
	);
	
	wp_nonce_field( 'wpse32973_nonce', 'wpse32973_nonce', false );
	echo '<select name="wpse32973_layout">';
	foreach( $opts as $val => $label )
	{
		echo '<option ' . selected( $val, $meta, false ) . ' value="' . esc_attr( $val ) . '">' . esc_html( $label ) . '</option>';
	}
	echo '</select>';
}

add_action( 'edit_post', 'wpse32973_save' );
function wpse32973_save( $post_id )
{
	if( ! isset( $_POST['wpse32973_nonce'] ) || ! wp_verify_nonce( $_POST['wpse32973_nonce'], 'wpse32973_nonce' ) )
		return;
	
	if( ! current_user_can( 'edit_post', $post_id ) ) 
		return;
	
	if( isset( $_POST['wpse32973_layout'] ) )
		update_post_meta( $post_id, '_wpse32973_layout', esc_attr( strip_tags( $_POST['wpse32973_layout'] ) ) );
}

add_filter( 'post_class', 'wpse32973_post_class', 10, 3 );
function wpse32973_post_class( $classes, $class, $post_id )
{
	$layout = get_post_meta( $post_id, '_wpse32973_layout', true );
	if( ! empty( $layout ) )
	{
		$classes[] = esc_attr( $layout );
	}
	return $classes;
}