I had to turn on moderation for comments on older posts and Captcha verification for comments in general because of an upsurge in spam posts, mostly advertising one specific website. For a long while, I got away with just requiring OpenID, which was convenient and I think encourages more posting. Blogger's spam filters caught almost everything before, but they seem to not have learned that comments with this particular URL are always spam. I guess I should feel honored that my blog is doing well enough to attract so much interest from page rank exploiters.
The moderation on older posts is also there because, when I looked through the spam trap, I found a couple legitimate comments on fairly old posts. The commenters were all regulars here on my blog, but they languished in spam prison because I wasn't looking for comments on old posts. Actually, I think I remember seeing at least one or two of them before, so I'm not sure how they wound up marked as spam. Anyways, moderation on posts older than 4 days should help me catch stuff I might otherwise miss.
On the subject of spam, I noticed that yellowdingo's blog, which had gone quiet for many months because of family issues, has now been taken over by a spammer, so I've had to remove it from my blog feed. I'm not sure how spammers figure out which inactive blogs still have lots of links or followers, but I've seen it happen a couple times before. They probably don't care about followers, per se, more likely links from blogrolls. Maybe I should check mine for dead links. Do not feed the spammers.