For you people who do your own theme work:

<?php if (function_exists('wp_get_shortlink')) { ?>
<div><span class="post-shortlink">Shortlink: 
<input type='text' value='<?php echo wp_get_shortlink(get_the_ID()); ?>' onclick='this.focus(); this.select();' />
</span></div>
<?php } ?>

Basically that adds a little Shortlink: input box with the shortlink in it. Click the box, and the shortlink becomes selected for easy copy and paste.

It’ll also work with any shortlink plugin that uses the “get_shortlink” filter to override the shortlink properly (like WordPress.com Stats).

Put it somewhere in The Loop or on the Single Post pages or what have you.