It's being rolled out to Direct Messages first, where much of the spamming occurs.
All links sent through Direct Messages will be submitted to this service. They'll be checked for spam and then shortened using a twt.tl root link.
Twitter says that even if a bad link has already been sent out through a Direct Message, if a user clicks, Twitter will be able to protect them.
Spam has been a problem for Twitter. If you're a regular user of the service, I'm sure you've had your fair share of Direct Message spam. There's also a ton of spam in public tweets and @ replies. Hopefully, this solution works for Twitter and they'll be able to provide better spam protection for the main Twitter feeds. Spam is a social network killer and one of the reasons for MySpace's demise.
Now, if we can only get Justin Beiber to stop being a trending topic.