The Twitter Blog announced they are now testing expanding their Twitter based URL shortening service, t.co on URLs that are long. The main difference here is that they will show a portion of the real URL, but pass it through t.co for security reasons.
When this is rolled out more broadly to users this summer, all links shared on Twitter.com or third-party apps will be wrapped with a t.co URL. A really long link such as http://www.amazon.com/Delivering-Happiness-Profits-Passion-Purpose/dp/0446563048 might be wrapped as http://t.co/DRo0trj for display on SMS, but it could be displayed to web or application users as amazon.com/Delivering- or as the whole URL or page title. Ultimately, we want to display links in a way that removes the obscurity of shortened link and lets you know where a link will take you.
There is also an advertising reason for this:
In addition to a better user experience and increased safety, routing links through this service will eventually contribute to the metrics behind our Promoted Tweets platform and provide an important quality signal for our Resonance algorithm—the way we determine if a Tweet is relevant and interesting to users. We are also looking to provide services that make use of this data, an example would be analytics within our eventual commercial accounts service.
For more information, see the Twitter Blog. Also see our related article, Twitter Gets Its Own URL Shortener To Stop Scams; Good Marketers Need Not Fear.