Warning - Arduino NG and SPI
If you're working with an Arduino NG and an SPI controlled device you're working with is not functioning properly (an AD5206, an accelerometer, etc), you'll need to perform surgery on your 'NG to remove the SMD LED from digital pin 13. Idiscovered this while helping YouJeong troubleshoot her AD5206. We looked at everything from the wiring to the AD5206 chips to source code to finally the Arduino itself. The only reason I was able to figure this out is that I saw that YouJeong's Arduino NG had an LED on pin 13 (which is one of the pins Arduino uses for its SPI interface). I noticed when trying the most basic "blink the LED" program that an LED inserted between digital pin 13 and ground was very dim. When I jumped it in parallel with the SMD LED on the Arduino circuit board it was brighter. Using a multimeter we found that the pin 13 was only giving us 1.92V when pin 13 was set HIGH.
I suggested we search for "Arduino NG SPI" and we found a thread in the Arduino forums about this issue. To rectify the problem, I removed the pin 13 LED from both of her Arduino NG boards and SPI started working properly.