I am connecting my XBee to an Arduino UNO via the Sparkfun XBee Explorer USB board. Can I supply power from the Arduino to the XBee via the 5V pin on the Sparkfun Explorer?
Yes, but the USB socket on the XBee explorer USB board should not be connected. You don’t want the Arduino uno feeding 5 volt back into the USB port of the Explorer USB. So no data communication with a computer along that route either.
Be aware though that connecting the UNO’s TX pin to the Din of the Explorer board requires levelshifting from 5 volt to 3.3 volt. Directly connecting TX to Din could damage the Xbee. The Arduino’s RX pin won’t be damaged by the Dout signal from the Xbee if connected directly. But the Dout voltage may not be high enough to be registered as a logic high reliably.