Using the bitcoin wallet PKI for message encryption
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.