Other forums that I am on put a black dot (or other symbol) next to the threads that I have posted in.
I think that if possible that it would be a good idea for BC.
I like the feature because when I just want to browse quickly through here I normally only read the threads that I post in. But since I read almost every thread I forget which I have posted in and which I haven't.
I don't think that phpbb has that available anyhow, and if they do then it is a mod that has to be added by Jeff. I haven't even looked at adding it to the forums I run, cause people get emails for any response that is posted in a thread they either started or replied to.