Most Wordpress bots are easy to spot. They promise cheap drugs, or wang enlargement, or spurt out gibberish and its evident that it isn't a real comment. You have to be pretty dense to not separate real post from these bots. There are other bots that offer some sort of nondescript advice or encouragement. These are a bit harder to spot, but still usually pretty easily spotted.
I approved a bot's comment a few days ago. The bot's comment was super specific and on point and was generally a great comment on the post. So great in fact that it already existed, but from a different commenter. This bot had read another commenter's comment and then reposted it using a real email address and believable user name, but set the user's URL to a spam site.
By no means do I like or condone spamming blogs, but i thought this was genius in a sense. More then likely what they did was scan for blog like any other bot would, but then they would utilize a screen scraper to pull down a blog post and copy the contents of the body of the post. From there it's the same as always, just posting a new comment, but substituting the body for the scraped one.
This spam bot is simple, elegant, and not widely used. I'm willing to bet that the people that are using this kind of spam bot are getting tons of inbound links to their sites because no one is noticing these spam posts.
Be aware next time you're filtering comments on your blog, there is now another kind of spam to filter out.