There's not much technical information on the Mindstorms NXT system yet. Some basic pictures and videos are available from its unveiling at the 2006 Consumer Electronics Show (CES) in Las Vegas. The price is being reported as US$249.99.
One big change from the earlier Mindstorms Robotics Invention System (RIS 1.0/1.5/2.0) is the use of LEGO Technics parts versus the LEGO bricks parts. New sensor-types, a change to servo motors, and the redesigned controller, the NXT Intelligent Brick, all seem to point to a move by LEGO to take the Mindstorms system to a completely new level.
Examining some online posts on the official LEGO community forums seems to indicate mixed feelings about this change, with the major concern being compatibility between the older RIS systems and the new NXT system.