Pavel Bánský [1] explains how he did connect the Digi XBee module [2] to the Tahoe development board [3]:
"XBee module is very sophisticated device controlled over serial link. It can operate in old fashion AT command mode as well as more advanced API mode. In the "robotic vehicle" case it's used as transparent point-to-point connection operated via AT commands. There are development boards for XBee with standard 9-pin DSUB which comes with jump-start-kit. Even if these can be easily connected to Tahoe serial port UART1, I've decided to go with my own daughterboard connected to UART2."
Read the full story about how to connect XBee module to Tahoe [4]...