Automattic's Wordpress.com has launched just exactly this: its own built-in short-link generator. When you're creating a post on the Wordpress.com service, you get an option to create a wp.me link alongside the post's default link.

The big advantage to these links, over links from third parties, is that they are pretty much guaranteed to work as long as the Wordpress.com system lives. There's no additional point of failure you introduce by using one of these links. They're also really easy to generate -- you get a short link as you're writing your post.

More: http://news.cnet.com/8301-19882_3-10314587-250.html