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.