imo, the phrase "public key" itself is confusing. Instinctively, how can a public key that everybody has possibly be secure? The solution - Use the word "lock" instead: Need to lock a document to send to me? Use a copy of my lock - here's one (a padlock icon would make sense in a gui). I've got the master key that unlocks all these padlocks.

There's also huge potential to be had in linking social networks into webs of trust: e.g. facebook groups whose members have signed each other's locks/public keys; or on something like facebook, though less broken, you don't get to be my friend unless we can sign each other's locks.