I think that any static content tends to get overlooked and become semi-invisible to people over time. But, there’s not a lot you can do about it other than make the rules accessible to those who remember to check them and visible to those on the forums for the first time. If it were up to me, I’d probably also consider adding a link that says “Forum Rules” down at the bottom of the page (down where the links to About, Blog, etc are) next to the Forums link. But I’m sure people would overlook that as well.