Here’s what I would propose.
Owners of nubit receiving addresses should be able to exchange messages over the block chain. The messages would be held in the block chain for 30 days, after which all nodes should delete the message from their copy of the block chain to save space.
The PGP private key should be deterministically generated from the nubits private key. The resulting PGP public key should be stored in the block chain with a special transaction. It should be optional to enable PGP message receiving for your nubits receiving address. When the recipient hasn’t stored their PGP public key on the block chain, it would be impossible to send them a message over the block chain.
The wallet client would have to have the necessary functionality for this to work.
How this would be useful? Easy, payment details. Often times merchants want to know more than just the amount and the TX inputs. Or, alternatively, people who don’t care about currency TXs would use nubits as their private e-mail client. We could ask higher fees for sending messages and have a new revenue stream.
The conventional e-mail system is still lacking native PGP support and Bitcoin’s approach to use HTTPS and a publicly known central server for attaching a message to your payment is IMO a complete failure. What if the merchant has no publicly available centralised server and a HTTPS certificate?