Attaching messages to transactions that get stored in the blockchain is highly wasteful and won't be implemented.
However, what will be possible in the next version is something like you want: Instead of making a payment by taking the payment address, creating a transaction and broadcasting it yourself, you'll instead be able to create the transaction and wrap it up with some other information like a message to the receiver, and then you send this whole thing to the receiver directly (like by submitting it to their website). The receiver then has the choice whether to accept the payment in which case he broadcasts it himself, or declines the payment, either way sending you a reply to tell you whether it's accepted or declined and additionally a message.
