For some time now, the direct messages portion of Twitter has been a hinderance of private communication for those who put it to use regularly. The company has been working on ways to make it a friendlier experience and are now presenting the obvious solution—the universal 140 character limit will be dropped in favor for the private message approach of other services.
Public tweets will remain the 140 character blast since the speed of a time line moves in real-time and usually its a lot of content to take in and respond to at any given time. Meanwhile DMs have always been like PMs on a message board where content doesn’t move as rapidly as your live time line and usually the matter of such messages can be attended to when you wish. At times this can require more characters to work with that the 140 tweet format.
Twitter wasn’t clear on when exactly the restriction on character-count in DMs will be lifted, but gave a window of sometime in July.