Using the bitcoin wallet PKI for message encryption Is it possible for a message sender to encrypt a message string with a receiver's public wallet address, email this ciphertext to the receiver and then have the receiver decrypt the message using their wallet (private key)?

I'm not talking about spamming the blockchain, the message could be sent using IM/Email once encrypted.

So it's sort of like using gpg but with the wallet public/private key.

Is this possible?
The cryptography Bitcoin uses for wallet private keys is ECDSA, using curve secp256k1:

http://en.bitcoin.it/wiki/Elliptic_Curve_Digital_Signature_Algorithm

It is for signing messages, and does not useful for any method of encryption.

There are other methods of encrypting and communicating a message, ... Gli.ph, BitMessage, GPG/OpenPGP, and more.

If you trusted the party you are sending funds to, that party could use the message signing capability in the client to sign a message that tells you what GPG key to use in encrypting a message to that party, for instance.  That would give you the assurance that you are encrypting a message truly only readable by the party that has control of that Bitcoin address and signed the message.