Michele, is there any plugin which can simply nofollow a new user's sig links until they hit the magical forty posts mark? Doesn't conditional signatures do this?
I understand your reasons for implementing this control but I feel it's too strict. Not everyone who signs up has the intention of spamming or abusing sig links. There are a few users on here who seem to be quitely annoyed at not being able to share their website links with fellow members.
One of the first things I do when I sign up to a new forum is to setup my signature. New users here are straight away being alienated from fellow members. A nofollow of sig links until they hit a certain amount of posts would be a positive measure IMO.