http://www.theguardian.com/science/2013/sep/12/voyager-1-leaves-solar-system
http://www.nasa.gov/mission_pages/voyager/voyager20130912.html#.UjNLFj-Gfc4
The discovery was due in part to the increased pressure of plasma from a recent coronal mass ejection from the sun and how it effected the probe, which could only occur if the probe was already in interstellar space.
The second Voyager is about 2 years away from leaving the solar system as well.