It was actually pretty easy to convert my previous example to the 2560; I just changed the port in setSerial() to Serial3 instead of newsoftserial and it worked. This left me a little time to play with capturing an I/O packet and taking it apart. The code to handle an I/O packet is in this example.
I wanted the I/O packet because I have an XBee outside on a pole measuring the temperature that sends I/O packets periodically and I want to set up a battery monitor for my lead acid batteries to keep track of their state of charge. You don't need an Arduino hooked to an XBee for these purposes, but you do need to be able to receive and decode the packets.
Once again, feel free to grab the code and modify it for your purposes.