This will only work for average email users if you can pull it off without ever using any of the industry language, or requiring anybody to ever actually do anything with a key. Find other descriptive language to use, and make it require zero extra effort, and you've got a winner.
We learn all the time how to do complicated things on the internet. Facebook isn't instantly trivial to use (though it seems that way now that we know how to use it). Neither is Google+.
The whole problem with PGP is that it's not worth learning to use because it depends necessarily on network effects. If Gmail deployed it, the network effects problem would immediately disappear. At first it would only work within the online webclient, obviously, and enabling it would have big consequences for how/whether client-based access (IMAP and POP) worked.
