Another possibility would be to embed a text message in the transaction. There's no provision in the GUI for it, but the source code allows for it (it's just not implemented at the moment).
I did not know that you can send messages with your transactions.
As Gavin said, you can't. But I didn't say you could - I said the source code supports it. Subtle but important distinction. It would require modifications to pass messages in the prerequisite script passed to SignSignature. And, of course, modifications to see what's there.
Ah well, that is not exactly true. If I send these amounts to someone, 72.69, 76.76, 79.32, I have sent a message. Hopefully, they send one amount back to me for, 228.77 to reload my message machine.
Or tie amounts to dictionaries and voila, just another "slow" IM.