Birgitta Jonsdottir, one of those targeted by the subpoena, is being represented by the EFF. Jonsdottir is a member of the Icelandic parliament who assisted Wikileaks' in its release of a classified U.S. military video in April of 2010.
The original court order was faxed to Twitter on December 14. The order covers accounts linked to Wikileaks including its own account, those of Bradley Manning, the U.S. Army Pfc accused of leaking the classified documents to the whistleblower site; Jonsdottir; Wikileaks volunteer Jacob Appelbaum; Rop Gonggrijp; and, of course, Wikileaks chief Julian Assange.
Between December 14 and early January, Twitter asked the judge to unseal the order, and on January 5, the same judge who signed the first order ordered the original court order unsealed. Two days later, last Friday, Twitter sent email notifications to the holders of the accounts listed on the subpoena, telling them the company would respond to the order in 10 days, unless “we receive notice from you that a motion to quash the legal process has been filed or that this matter has been otherwise resolved.”
These sorts of orders are not uncommon, actually. It's also widely assumed that other companies, such as Facebook and Google, have received the same sort of court order for the same type of information, about the same users.
