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);