Using 6 axis accelerometers
From the Arduino playground:
This sensor board has a voltage regulator. When using 3.3V to the VCC the resulting voltage (after the onboard voltage regulator) might be too low for a good working I2C bus. It is preferred to apply 5V to the VCC pin of the sensor board. The board has pull-up resistors on the I2C-bus. The value of those pull-up resistors are sometimes 10k and sometimes 2k2. The 2k2 is rather low. If it is combined with other sensor board which have also pull-up resistors, the total pull-up impedance might be too low.