According to the Twitter developer documentation, Twitter plans to start wrapping all links with their t.co shortener starting in October. Currently, only ones over a certain length are wrapped.
So if you’re like me, and have a personal URL shortener that generated smaller links than t.co, you probably think this move is a little silly.
On the plus side, Twitter.com and the official Twitter clients (as well as any that use the right API methods) automatically expand all t.co links and display the URL they point to. So users should optimally never see the t.co URLs at all. If you continue to use your own shortener, it’s URLs will be re-wrapped, but users will still see your link.
Developers should check out the t.co URL Wrapper documentation page, as well as the wrap_links parameter in the statuses/update API method.