With popularity comes trouble. Ask any Windows user, or the Notorious BIG. As Twitter's popularity has skyrocketed, it's become the target for various scams, and the Twitter team is now taking action to stop the abuse.
The fight against spam is a consistent one for the Twitter team, but recently, phishing scams have started gaining in popularity. Spread mostly through direct messages (or email notifications about those messages), these scams offer a malignant link that, when clicked, can result in a whole mess of headaches for the user. From viruses, to stolen passwords, to remote access, these scams are a serious problem for novice Twitterers.
So in response, Twitter has set up a separate service that routes all links submitted to Twitter through a separate service, which detects bad links and wipes them clean. Quoth Twitter:
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.
Links will often be shortened to Twitter's own twt.tl domain, but the rest of the machinations of the new service will be undetectable. We'll see more from this service soon, but hopefully it works as promised and puts an end to the malicious Tweeting.