Swapnote — aka Nintendo Letter Box — is the 3DS messaging service, and it's out in Japan today.
Nintendo Japan's website has videos of the application in motion, and now we have a better understanding of how it'll work.
You can create notes of up to four pages long to be sent to a friend over the Internet (i.e. SpotPass) or locally via StreetPass. The recipient will see your note being drawn, though there's no animation abilities, so that'll have to wait until Flipnote Memo next year. You can add sound using the 3DS microphone or a photo taken with the camera, or use one of the built-in backgrounds unlocked by spending Play Coins.
It certainly seems this isn't an as-live chat function, hence the Letter Box name in Europe, and is more likely to be a WiiConnect24-style messaging service that encourages you to check your messages every so often. The site also shows how you can use the application as a diary.
Head over to the Nintendo Japan site to see Swapnote/Nintendo Letter Box in action.
Nintendo Letter Box will be available in Europe tomorrow. We'll know at 1pm UK time if it'll also be out in North America, so check for the Nintendo Download then.