I am sorry I did not think about mentioning that earlier. We generally try to mention common issues customers might run into, or bring up relevant information where applicable. However, since your system has not had a power issue up until recently, it does not seem unreasonable that I had yet to check to make sure you were powering all of your devices.
We put a lot of time and effort into having detailed documentation so our customers can find all the information they need. I am glad you figured out what the issue was by reading the user's guide.
To supply 3.3V power to your XBee without an mbed, you can use one of our step-down regulators to reduce the 3pi's battery voltage, VBAT, to 3.3V. It looks like the D24V5F3 should be able to provide enough current for a standard XBee.