Of course, if it was a substantial improvement over the other "Internet Phone" stuff that's out there, and had a good way of dealing with switching, etc., then people would use it to make "free" l.d. phone calls on the net, and the cryptography would get a free ride.
Generally, you are right in suggesting that anything that requires people to crawl behind their computers, attach new cables, purchase and debug a sound card under Windows, and generally engage in techno-weenie hardware manipulations will have less appeal than something plug and play. Even given the extremely user-hostile elements of PGP the software, I would be surprised if PGPFone became as popular.
Hmm, oportunities for 'consulting fees' abound in setting up PGP, PGPFone is another one. ;-)