This should help you on your way. Its got the schematics to build an arduino shield around an MT8870 DTMF decoder chip. Whenever a valid tone is detected it outputs the associated value in binary on four output pins. There is a fifth pin that strobes after each new tone.
If that doesnt do it for you, im sure this will. Its pretty thoroughly explained. Its an ATmega8535, but what ever. Its still an 8bit avr, so im sure you can brain together how to use an arduino instead.
