Think they're arranged by last post By default. They probably should just stick in the order they're stuck though. Feature request maybe.
I think it needs to be changed by admin on settings.
I would also like something like this for other reasons. We have a sub category where users post questions and we have certain people assigned to receive emails when new topics are posted - they are supposed to then go and reply to the question and it would be nice to be able inform them somehow that someone else (that was emailed) is already in the process of replying.