BitcoinTalk
We accept Bitcoins [moved to bitcoin.it/wiki/Trade]

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Can I just butt in with a question on why that is? To me it seems that if Bitcoin uses public-key cryptography to transfer ownership of the coins, it should be a trivial matter to include a short message that is only readable by the recipient.
Almost but not quite.  Bitcoin uses EC-DSA, which can only do digital signing, not encryption.  RSA can do both, but I didn't use it because it's an order of magnitude bigger and would have been impractical.