I have noticed recently that when you are sent a message in inbox, it has to: and from: information in it. What happens though, when you get a chain of back-and-forth communication, and then add a person to the Inbox Message, in those headers, it looks like they received the first message when it was actually sent (days before they were actually added to the conversation.) This could be potentially a big problem, when trying to track down who received what message, and when.
The idea I would like to put forward, is not changing the To: and From: in the previous messages and change it only for future ones, when people are added to the message. They would still be able to see the chain of messages, just their name wouldn't be in the previous headers.