Wireless Communication Ideas
Digi has XBee modules which have the ZigBee protocol built-in and present themselves to your AVR as a simple UART, easy to interface. Digi has point-to-multipoint and mesh solutions. Modules are compact and reasonably priced.  

Running off a coin cell may not be possible; RF transceivers are rather power-hungry (10s of milliAmps). Unless you have a very small power duty cycle. If for instance the device is switched on for 1 second every ten minutes you have a duty cycle of 0.17%.