Post Meta Revisions

gistfile1.php

<?php
/*
Plugin Name: Post Meta Revisions
Description: Revisions for the 'foo' post meta field
Version:     1.0
Author:      John Blackbourn
Plugin URI:  http://lud.icro.us/post-meta-revisions-wordpress
*/


function pmr_fields( $fields ) {

  $fields['foo'] = 'Foo';
	return $fields;

}

function pmr_field( $value, $field ) {

	global $revision;
	return get_metadata( 'post', $revision->ID, $field, true );

}

function pmr_restore_revision( $post_id, $revision_id ) {

	$post     = get_post( $post_id );
	$revision = get_post( $revision_id );
	$meta     = get_metadata( 'post', $revision->ID, 'foo', true );

	if ( false === $meta )
		delete_post_meta( $post_id, 'foo' );
	else
		update_post_meta( $post_id, 'foo', $meta );

}

function pmr_save_post( $post_id, $post ) {

	if ( $parent_id = wp_is_post_revision( $post_id ) ) {

		$meta = get_post_meta( $parent_id, 'foo', true );

		if ( false !== $meta )
			add_metadata( 'post', $post_id, 'foo', $meta );

	}

}

add_filter( '_wp_post_revision_field_foo', 'pmr_field', 10, 2 );
add_action( 'save_post',                   'pmr_save_post', 10, 2 );
add_action( 'wp_restore_post_revision',    'pmr_restore_revision', 10, 2 );
add_filter( '_wp_post_revision_fields',    'pmr_fields' );

?>