Prosecutors need a warrant to subpoena tweets, even if deleted, because they're public information owned by a third party, a New York judge ruled on Monday.

The case is People v. Harris, Criminal Court of the City of New York, No. 2011NY080152.

Your Tweets Can Be Subpoenaed - Yahoo! News

This is certainly something to think about when using social media. and now tweets do not have 4th amendment protection.