It still has some kinks, but it is a good start. Generally, Twitter had acted to protect users by banning those that spread links to malicious sites. This is has significant problems, notably it being to late for the first to click on the links/tweets and the ease of creating a new account. They turned to Google for help in their efforts (http://blogs.zdnet.com/security/?p=3880).
"Whenever a Twitter user is attempting to post a link to a known malware/phishing URL, a “Oops! Your tweet contained a URL to a known malware site!” message will appear, and prevent this from happening.
Does the feature really work? A five minute test showed disappointing results, making it obvious that it’s still in experimental mode."