How to show wp.me shortlink underneath each post?

WP 3.0 includes:

wp_get_shortlink()

which returns the ?p=1 style url for any post within your domain. This can be filtered, however for other shortening services, which is what the WordPress.com Stats plugin does for self-hosted blogs. With that plugin installed and activated, wp.me shortlinks will be output instead of the default mydomain.com/?p=1, and can be retrieved with the function which can be used within the loop as:

echo wp_get_shortlink()

or to retrieve any post outside the loop:

$link = wp_get_shortlink($post->ID);