I've written an Arduino XBee library and posted it on google code http://code.google.com/p/xbee-arduino/
This is for communicating with XBees in API (packet) mode. You don't need any special software for AT (transparent) mode.
Currently it supports sending/receiving packets for both series 1 and 2 XBee and I plan to expand the API support soon. I have provided a few examples and tested on both series 1 and 2 radios.
BTW, this is my first Arduino library and first C++ app so I'd appreciate any feedback.