Here is a simple bit of code you can drop into your functions.php file then use in any of your templates where you want to display a short statement telling when a post was last modified.

// Modified Post - requires WordPress version 3.0 or greater
function modified_post(){
  global $wp_version;
  if (version_compare($wp_version, "2.999", ">")) {
    /* If the post date and the last modified date are different display modified post details */
    if ( get_the_date() <> get_the_modified_date() ) {
      echo '<div class="modified-post">'; /* CSS wrapper for modified post details */
      echo 'Last modified by ' . get_the_modified_author() . ' on ' . get_the_modified_date();
      echo '</div><!-- .modified-post -->';

Of course, you may have noticed the check for WordPress version 3.0 or greater. This is required as the function get_the_date() on line 6 is currently only found in versions of WordPress starting at 3.0

Once the above code has been added to your functions.php file and you are using a version of WordPress greater than or equal to 3.0 then all you need to do is drop the following line of code inside the loop of the template file you want to display the modified post details on.

<?php modified_post(); ?>

Also included is a CSS class element for easy styling. The code can be easily edited to add more elements, too.

Now, what good is this little bit of code? Have a look around and see it in action.