wordpress.com intoduced wp.me short urls for hosted blogs, but you can also do this via your self hosted blog.
Install the wordpress.com stats plugin
http://wordpress.org/extend/plugins/stats/
You need a wordpress.com account, and use the API key once you have it.
The following code will generate your wp.me shortcode
Code:
<a href="<?php echo get_shortlink($post);?>">short link</a>
or you can get it by clicking generate shorturl
sample of it working ->
http://wp.me/pKjxA-1
enjoy.