See figure 4 in the datasheet). translate the 3.3V signals on the XBee side to and from the 5V signals on the Arduino (I *think* the RPi IOs run at 3.3V, so those can just be connected directly).

Good news DIYers, XBee now comes in WiFi flavors. The favorite wireless module of Arduino tinkerers everywhere is growing up and adding 802.11n to its normal compliment of RF and ZigBee options ...