Apple tend to put a lot of polish on things and an Apple mandated key service which can provide an authoritative list of who is trusted and such would be a big push to pgp everywhere.

not to mention the "click once, you can now sign your emails" UX would be lovely to have.

And with the icloud accounts system you have a method of having some (although minor) trust that the person making the private keys, is, who they say they are.

it would be very easy for apple to have a fully integrated pgp solution. :D