And here's a solid article about the reason a t.co URL shortener could be a very big deal for Twitter:
Cookies, long the basis for tracking users, need web browsers to store them. In a world where we share URLs via email and social networks, those cookies get lost along the way, and with them the ability to track viral spread of a message...
Every time you access the Internet through a particular app -- Facebook, Gowalla, Yelp, Foursquare, and so on -- you're surfing from within a walled garden. If you click on a link, all the marketer sees is a new visit. The referring URL is lost, and with it, the context of your visit.
This is why short URLs are so important. URLs survive the share... That's why URLs are the new cookies.