How are "message signatures" verified without having access to the public key? In the bitcoin-qt client you can sign and verify messages using only the public address. Since the public address is a RIPEMD160 hash how is it possible that I can verify a signature since I do not have access to the public key?
When signing a message (as opposed to signing a transaction), the signature contains additional information needed to recover the public key using the signature and the signed message.  The recovered public key is then hashed and the result compared to the address.  If they match, the message was signed by that address.