I've used various ZigBee implementation for myriad of commercial projects. I also started up the group:
**broken link removed** Embedded Wireless Networks
which I've noticed you recently joined. You'll find numerous example projects, tutorials and documentation concerning the ZigBee Protocol and its applications in the group.
After you begin the design of your project, if you have any particular questions post then in this thread and I will do my best to address them.
One issue I should mention, if you are looking for a wireless serial cable replacement, say from a master MCU node to a remote light control node, then the XBee Series 1 will be the simplest solution.
However, if you have a master node controlling several remote light control nodes, you'll need a mesh network and will need to use the XBee Series 2 or compatible devices.
The Series 2 require more initial setup compared to the Series 1 which will usually work right out of the box.
Also an excellent reference is the:
Building Wireless Sensor Networks: with ZigBee, XBee, Arduino, and Processing
The text will get you up and running fairly quickly, there are other very good books concerning the ZigBee Protocol, however they are more advanced and are usually not suitable for the first exposure to ZigBee.
BigDog