Andy,
looks like a nice project 
If you need more than 255 devices per channel you can just add an address to the packet data. In the old days the OSI protocol used 40 bytes address space and one of my collegues once calculated thats enough to address all objects in the universe 
Wake on radio (WOR) will reduce power usage on the CCx. By using interrupt receive on the atmega you can put the atmega to sleep as well, saving more power.
More details on sleeping can be found here: rubenlaguna.com/wp/2008/10/15/ar … the-usart/
And if you really want to read a lot on powersaving: news.jeelabs.org/tag/lowpower/
@Icing: should we extend rfbee sleep mode to letting the atmega sleep as well ?
We could define a ATSL to define sleepmode. E.g.
0- WOR for CCX, IDLE for at168 (radio reception will work, serial will still work)
1- WOR for CCx, STANDBY for at168 (radio reception will work,serial will take some time to wakeup)
2- SLEEP for CCx, SLEEP_MODE_PWR_DOWN for at168 (lowest power consumption, but longest wakeup time, WOR will not work)
etc
Cheers,
Hans
ps. “without a hitch” means “without a problem”