Sounds like we're on our way to reinventing S/MIME. Other than the
key signing and distribution (which I agree is a major can of worms)
it works remarkably well.
Which sounds kind of like, "Other than that Mrs. Lincoln, how was the play?"
Yes, and no. PGP and S/MIME each have their own key distribution
problems. With PGP, it's easy to invent a key, and hard to get other
people's software to trust it. With S/MIME it's harder to get a key,
but once you have one, the software is all happy.
The MUAs I use (Thunderbird, Alpine, Evolution) support S/MIME a lot
better than they support PGP. There's typically a one key command or
a button to turn signing and encryption on and off, and they all
automagically import the certs from on incoming mail.
R's,
John