monophonic/polyphonic retuning

>What about with Kelly Fitz' LemurPro for Mac? Still a load of work (and for
>me, a lot of waiting) but it would be _possible_ to retune instruments or
>their partials in a polyphonic file.

Well, first, let me never find myself in the position of saying
"never"...

The primary difficulty is that of
determining which partials go with which notes.
(This is necessary because otherwise you are scrambling up the
partials from different voices).
The process is simple enough to envision - e.g.,
find the fundamentals, then identify a harmonic series
above each fundamental (assuming the sounds have known
harmonic spectrum). However, it is very tricky to make work.

Im sure you are aware that all the commercially available
pitch detecting software (and hardware, like the antares)
works only on monophonic
sources - this is why, say, MIDI guitars need special pickups -
to isolate the sound from each string for separate processing
for the pitch detection process.

Bill Sethares