adafruit.com writes: [via]
Converting an Arduino to 3.3V – All official Arduinos run on 5 volts, which for a long time was the ‘standard’ voltage for hobbyist electronics and microcontrollers. But now the coolest new sensors, displays and chips are 3.3V and are not 5V compatible. For example, XBee radios, and SD cards and acellerometers all run on 3.3V logic and power. If you tried to connect to them with 5V you could damage the internals of the accessory. We use chips like the CD4050 to do level conversion but if you are using a lot of 3.3V devices, maybe you’re just better off upgrading the entire Arduino to run from 3.3V! To do that, we will replace the regulator so that the DC barrel jack for a 3.3v type, and then reconfigure the 5V usb power line so it goes through the regulator as well.
Converting an Arduino to 3.3V – [Link]