I have developed a few Arduino compatible boards for building robots. Take a look at my blog here: http://seriousrobotics.wordpress.com
Now here are a few questions on what are you trying to achieve with this project:
1. Do you want at least the same functionality as the NXT controller? I mean 3 motor ports, 4 sensor ports, LCD, buttons, speaker, USB, Bluetooth?
2. Do you want a shield that can be plugged over an Arduino and let people use the Lego motors and sensors or do you want a special made board?
3. How much do you think will be the resale price of such a board/shield?
Depending on the answer, different variants can be made. Let's say you want to replicate a NXT brick with an Arduino compatible controller or shield. Only the Arduino Mega has enough pins and functions to perfectly emulate a NXT brick. The sensor ports on the NXT have one analog pin and 2 digital pins. Times 4, that's 4 analog pins plus 8 digital pins. One port has I2C on the 2 digital pins, so I guess there are only 6 digital pins used. I will have to look closely at the NXT brick schematic to know more. The motor ports have 2 wires that control the motor and 2 wires that read the quadrature encoder. For each motor there is an H-bridge that uses 1 PWM pin and 2 digital pins from the microcontroller, so that makes 9 pins (3 of them being PWM) plus 6 more pins for the encoders. The total is: 21 digital pins, 4 analog and 2 for I2C. Then there are the buttons, the LCD uses the SPI interface and the USB uses the hardware serial interface and another hardware serial interface for the Bluetooth module. So the Mega can surely cover all the pins needed and have spare. But perhaps will not be able to run everything smoothly. The NXT has an ATmega8 that takes care of the motors and encoders and talks to the ARM controller through the I2C interface. So yeah, if you get all the parts and put them together to make a similar controller you end up having a product that will cost about the same as the NXT controller sold separately ($150).
So it all depends what you want to have. To function properly, the NXT sensors and motors need several pins from the microcontroller. Depending how many sensors/motors you want to use with the Arduino, a design can be made based on the ATmega328 or on the ATmega2560.
Before the NXT came to market and before I ever heard about the Arduino I have made a couple of tiny controllers for the old RIS motors and sensors, using a Nemesis and a Perseus controllers (based on PICs) made by Kronos Robotics. I could connect 2 active sensors, 2 motors on both controllers, plus the Nemesis has 2 connectors for a servo and a Sharp IR sensor. The Nemesis also had a dual IR proximity sensor, the Perseus had only a IR receiver that allowed me to remote control it using a TV remote. The Nemesis was housed in a 2 layer 4x4 brick plus base plate, the Perseus was housed in a 2 layer 2x4 brick plus 2 base plates. The page I wrote at the time does not exist anymore, but here are some pictures with robots built with them:http://seriousrobotics.wordpress.com/lego-robots/mini-sumo/http://seriousrobotics.wordpress.com/lego-robots/line-following/
So yeah, the old RIS parts are easy to use with Arduino. I can easily build a robot that uses 2 RIS motors and a bunch of RIS sensors, also some RC servos using the Robot Builder's Shield or the Robot Builder's R-duino that I designed.