I think reaching the screen where I can PM a person from a given thread requires too many clicks. I think it will be much, much easier from a user experience standpoint if there was an option to PM a person while I am in a thread, scrolling past his post. E.g. if I hover his avatar or username, if there was a dropdown/contextual menu with the most helpful options related to this user - this would be great.
By clicking on a members avatar or userID you bring up a screen with a link reading start a conversation.