Nathan from Sparkfun gives an excellent demonstration of sending code to AVR chip wirelessly with Xbee and the Sparkfun wireless bootloader. As he explains the setup easily tolerates a break in connection during transmisson –

We’ve posted a new tutorial describing how to reliably download code to an AVR ATmega168 using a XBee serial link. We created our own bootloader and modified our old Screamer VB app to make a really robust wireless bootloader.

Wireless Bootloading for ATmega168