In an effort to rid the service of spam and malicious links, Twitter has announced that they are going to start scanning all the links through the site. It will be an impressive thing if they can pull it off without noticeably slowing down the site.
"Today, we’re launching a new service to protect users that strikes a major blow against phishing and other deceitful attacks. By routing all links submitted to Twitter through this new service, we can detect, intercept, and prevent the spread of bad links across all of Twitter. Even if a bad link is already sent out in an email notification and somebody clicks on it, we'll be able keep that user safe.
Since these attacks occur primarily on Direct Messages and email notifications about Direct Messages, this is where we have focused our initial efforts. For the most part, you will not notice this feature because it works behind the scenes but you may notice links shortened to twt.tl in Direct Messages and email notifications."