I've been getting spammed regularly these last couple of months. By the end of this week, I'll have seven years worth of posts archived here. I'll never live long enough to clean all the spam out of them. In fact, I just found spam on a bunch of the posts from my very first week in March, 2002.
So we've had to add moderation to the comments on all posts more than seven days old. Why seven? That's about how many posts turn up on the first screen, and I'll easily see if I have any spam there.
This will mean that late comers to the party won't be able to see their comments come up right away. However, it also means that I'll see their comments. Sometimes I realize that someone has commented on a two or three week old post, and I ended up missing it. So that, at least, will be fun for me.