And what if PGP were modded to use combinations of words out of the dictionary (human readable) as public keys? I have no idea how that would even be possible, but it seems to be theoretically possible, and that would satisfy all three requirements: security, decentralization, and human-meaningful.

I probably missed something.

What is human-meaningful about destructively omnipresence stiff fathom bushwhacked parasailing jumbotron articulated sline comonad ephemeral xenophile butt headphones liquid turquoise soar classic pantheon?

I can read it to you over the phone? Or even more simply... I can just read it

Marc Stiegler's essay on Petname systems is a far better introduction (it's listed in the external links). http://www.skyhunter.com/marcs/petnames/IntroPetNames.html

He uses the term memorable rather than human-meaningful, which is a more accurate description I think.

Dictionary words can get mixed up on the phone. I sometimes find myself asking for words to be spelled out when I talk on the phone.

In this context, human-meaningful really means human-chosen. With public-key crypto, you don't get to choose what your public key is.

That would require either a drastic reduction in the size of the key space, or a drastic increase in key size.