That's a popular request. I, for one, don't like it though... it will cause a lot of people to post to "none" making it impossible to ferret out a thread using the "In reply to" and "Replies (1)" buttons. The thread of argumentation will be broken for lurkers.
Also, people will be able to tell who I have on ignore just by trying to reply to one of my posts. I'd rather my ignore list be private.
If Bob could just prevent posts of ignored people from showing in my mailbox, that would be cool, but it would be nice if they could still reply publicly.