The real-time platform has been routing links within Direct Messages through its own link shortening service to detect, intercept and prevent "the spread of malware, phishing, and other dangers" but they were previously wrapped with a twt.tl URL. Security is helpful and all but Twitter clearly has more reasons than one to own the URL shortening space entirely.
When the service launches for all tweets more broadly this summer, users will have a way to automatically shorten URLs when posted directly through Twitter. What is important is that routing links through their own system will eventually contribute to the metrics behind Twitter's Promoted Tweets Platform.
The real-time Web just got a lot more competitive with the introduction of Twitters T.CO and the future of competing URL shortening services could be in serious jeopardy. If you are using a URL shortener you will be able to continue using it, but Twitter plans to wrap the shortened links you submit.
Developers who create applications on the Twitter platform get first crack at the service. They will be able to choose how to display the wrapped links in a manner that is "most useful, informative, and appropriate for a given device or application," according to Twitter.