Circuit Cellar http://www.circuitcellar.com has the first part of a two-part tutorial on the ZigBee standard in the February 2005 issue. "Zeroing In On ZigBee"
Looks to be pretty involved. This month is discussion of the layers to the protocol and next month is discussion of the physical implementation.
This is the leadup to the upcoming Circuit Cellar ZigBee design contest for those interested.