Here are some Arduino / AVR links related to Ham radio / APRS / TNC / AX.25 stuff :-
Some projects are not updated or seems dated ...
BeRTOS examples with APRS :-
The below three are based on the codes from BeRTOS ...
AFSK 1200 modem
AX.25 Packet Modem
Arduino KISS TNC
extdigi, an APRS Digipeater for Arduino
http://extradio.sourceforge.net/extdigi.html
I'm getting the parts to build the AFSK 1200 modem mentioned above..
Stanley ( 9W2SVT )