'by integrating various pieces of code and tables of data, which are all (I believe) in the public domain.'
It uses either the Carnegie Mellon Pronouncing Dictionary [cmudict.0.?] (Copyright 1993 by Carnegie Mellon University), or (by default) "beep", a direct desendant of CUVOLAD (british pronounciation) which was used by previous releases of rsynth, which has a more restrictive copyright than CMU dictionary.
This Say... WWW interface to rsynth ('UNIX pipe-fitting') was created by Axel Belinfante.