> Does anyone know of any ap-notes or examples using a PIC to read DTMF tones?
>  Or would I be better off using a tone decoder chip such as the 75T204?

I have done DTMF decoding using just a zero-cross detector and a 20MHz
PIC.  Depending upon whether tone generation is required as well, the
routine uses about 40-70% CPU time [it has two parts: an interrupt routine
that uses about 40-50% CPU, and a calculation routine that needs to be
called about 60 times/sec].

Unfortunately, because I developed the routine for my employer I cannot
release it on the net, at least not presently.  On the other hand, I will
say that it is possible to do a good job of detecting DTMF or other such
tones, using a PIC, even in the presence of other audio (such as dial
tone).  My routine looks for eight tones simultaneously, and is selectable
between two groups of eight (one set up for DTMF, one for call-progress
and special tones).