Twitter released automatic link shortening on Twitter.com, according to Twitter blog.

When a user pastes a link of any length into the Tweet box on after composing Tweet and hitting the “Tweet” button, the link will shorten so that it only takes up 19 characters.

Plus, since a shortened version of the original link is shown, people will know which site the link points to. This service also increases security. If users click links that are reported as malicious, Twitter directs them to a page that warns them.