are now soulbound. And that's a huge deal, apparently, to roleplayers -- they used those little scraps of paper for everything from treasure hunts and "official" documents to invitations and tickets.
Now, I'm not quite sure exactly why this change would be made -- for what possible reason would you want to keep people from trading saved copies of in-game mail? It's not like it would stop anyone's communication -- anything goldsellers could say in these sheets of paper, they could say in in-game chat just as well. And I can't think of any possible way that these saved letters could be used to do something devious like pass secret letters or communicate across factions-- there's no reason to make them soulbound by default.
Of course, it may just be a bug -- that's what someone was told by a GM, so maybe Blizzard will put a fix in, and RPers can have their murder mysteries again. Unfortunately, I can't seeing it justifying a hotfix -- odds are that even if it is just a bug, RPers will be waiting quite a while until they can trade letters again.