Twitter is updating the way their t.co link shortener works and the end result is that users who share a link in a Tweet will have 2 fewer available characters for the rest of the message. The shortened links will now be 2 characters longer, meaning http links will now take up 22 characters and https URLs will gobble up 23 characters.
Twitter hasn't specified why they are making this change, but they did say "we made this change for a variety of reasons." I would guess that so many links are being shared on Twitter that they were running out of URLs to use. It's possible that there are further reasons behind the change, but those are not immediately clear.
The official change for this isn't until February 20, 2013, though Twitter alerted third-party developers yesterday so that they could ready their applications, if need be.