t.co is now in full force. And boy, do they have major issues. Yesterday, Mon, Nov 21, 2011 is when the conversion took place. All links on Twitter were t.co. Thing is, the hover (title) was not showing the destination URI so you had no idea where the link resolved to.
Just tested again this morning and it appears that some are resolving, some aren't (showing destination URI on hover). I wonder how many are going to get Rick Rolled today? I got rolled yesterday by someone testing the new t.co features.