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.
User groups in categories permissions
In the access control in the admin area you have "read" & "write" against the groups, when I click on the read & write options they go grey but do not appear to change my ability to read or write to that particular category, is this purely because I am a admin or am I missing the point?
Yep, as an admin, you automatically have global access to all categories.
You can create a new unprivileged user to see how it interacts with regular users.
@julian thank you will have a play, but is the access default read & write on? as in white and too restrict grey?
No, the default is "off", but keep in mind that if a category has NO restrictions at all, then it is globally accessible.