Infos from TheNxtStep (Author: Daniele Benedettelli):

MINDSENSORS has designed this shield that will allow Arduino to replace *ehm* to be integrated into the LEGO MINDSTORMS system.

It will have 4 NXT motor ports, 4 NXT sensor ports and several RC servo motor ports. The NXShield will attach to an Arduino Duemilanove or Uno or similar Arduino clones. (Picture shown here with a Duemilanove)
The programming for the NXShield will be on Arduino, using usual Arduino C/C++ programming environment. There will be only one I2C port, so you can use multiple sensors on the bus only if they have different addresses.