Twitter recently posted, Trust and Safety, a blog entry that discussed the steps they are taken to prevent scams and malicious links on Twitter. This was in response to the recent Barracuda Labs report that showed in October 2009, one in eight accounts created were deemed to be malicious, suspicious, or otherwise misused. But at the end of the Twitter post there is mention that they are going to be redirecting links within Direct Messages and email notifications. It looks like they will be using the URL shortener twt.tl to do this. It seems to be open ended as whether they plan to roll this shortening service out to status updates.
Danny Sullivan, Search Engine Land, put the Twitter URL shortener to the test to see if the tracking of the originial shorten URL would be lost. He found that there indeed seems to be NO impact.