Twitter has established a new policy for dealing with copyright violation claims in tweets. It may look ominous at first glance, but a new message that can appear in your Twitter timeline, which reads "This Tweet ... has been withheld," is actually an improvement over the company's previous response, which was simply to delete the tweet in question.
The updated policy, first reported yesterday, replaces the potentially offending tweet with a placeholder message that links to Twitter's new Copyright and DMCA Policy page, which explains what recourse you have when a person or corporation files a DMCA "takedown" against your tweet or image. (And because it would be hard to violate copyright in just 140 characters, it is probable that most DMCA takedown requests sent to Twitter will be for images, not text.)
The DMCA — Digital Millennium Copyright Act — protects content hosts like Twitter or YouTube against lawsuits, provided they act promptly to remove copyrighted material when the rights holder files a claim. (It is subsequently up to the individual uploader to disprove the takedown claim, if he or she is able.)
It's an imperfect system for users, but Twitter's new policy change is a step in the right direction. Better for a tweet to be "withdrawn" than removed forever.