Looks like they're going to wrap existing shortened URLs with t.co:
If you are already partial to a particular shortener when you tweet, you can continue to use it for link shortening and analytics as you normally would, and we'll wrap the shortened links you submit.
If you are already partial to a particular shortener when you tweet, you can continue to use it for link shortening and analytics as you normally would, and we'll wrap the shortened links you submit.
http://blog.twitter.com/2010/06/links-and-twitter-length-sho...