This REALLY simple plugin allows you to use a short URL to your post. It also redirects any page loads of "index.php?p=#" to the permalink of the post.
This plugin is also smart enough to not try and redirect any page loads of "index.php?p=#" when you aren't using permalinks. (Although there is really no need to be using this plugin if you aren't using permalinks as your URL is already about as short as it can get.)
Basically, I wanted like ?p=#, but that wasn't ugly.
Click here to see it in action: http://viper007bond.com/post/38. Clicking that link will take you to a post about WordPress v1.5.2 being released and note that the URL in your address bar is now the full permalink.
Also try visiting this link that I don't want people really using: http://viper007bond.com/index.php?p=38
The "38" is the post ID of that post. Easiest way to find the ID of your post is to hover over your edit link, or if you don't have one, look on the manage page in your WordPress admin.
You could also place some code like this somewhere in The Loop if you wanted to create short links on your site:
<small><a href="<?php bloginfo('url'); ?>/post/< ?php the_ID(); ?>" title="Short URL to this post">Short URL</a></small>
More details here: http://www.viper007bond.com/wordpress-plugins/short-urls/