I was curious about doing the DTMF in software; a google link returned this project that uses an ATMEGA1281 to do it in sfotware.
http://www.rtzaudio.com/kg4lne/micro-rc.asp
And it's by a KG4 ham operator.. KG4 must be the world centre for repeater/DTMF work!
Anyway, he has posted full code and schematics.
D