I spoke with some tech people regarding this issue and they said that MMS is an across the board problem that other manufacturers are also experiencing not just the Iphone. When sending a picture, it must be sent MMS. When you have more than one person on it, this MMS acts like an email or chat room where everyone can see and when replying, the setting automatically defaults to a reply all mode. That's why for some of these replies you se an RE: or a subject.
This Christmas when I send out multpile texts, I'll be sure to have MMS and group messaging off as well as iMessage. That is supposed to do the trick as long as what's being sent isn't considered an MMS.
iMessage acts the same way. I was told by Apple support that multiple recipients on a broadcast iMessage is like opening up your own chat room...just like MMS.
I hope they get a fix soon too.