I'm still experiencing the problem now and then. Today I discovered something interesting.
I have two messages lying around in my Sent box, since their attachments cannot be removed. I just got an answer to one of them, and suddenly I could remove the attachment from the original sent message. The other message, still unanswered, cannot be relieved from its attachment. There seems to be some connection here - or was it just coincidence?
I know this is a user forum, but if Apple means anything with their usability image, someone should monitor this conversation (and others) for input to problem solving and product enhancement. Hey, Apple?