Lego has posted an FAQ addressing some common questions about the new EV3 set. One of the key points is the compatibility between the new EV3 and existing NXT motors and sensors. The good news is that your existing NXT motors will work with the new EV3 brick, and the new EV3 motors can be connected to the NXT. However the new EV3 sensors are not backward compatible with the NXT.
Read more here at the Lego website.