As if bloggers don't already get enough comment spam, now spammers can Buy Blog Comments. I think most bloggers are pretty diligent about removing comments that are worthless or seem to be purely for search engine ranking purposes, so I'm not sure it's worth the money to use this service. And anyway, if you do things just for SEO, you are playing a short-term game. In the long run, it's better just to leave comments that are useful, insightful, or somehow add to the conversation.
As an example, Chris from Direct Text Book leaves comments here sometimes. He leaves the URL of his business on his comments. I'm fine with that because he doesn't just randomly comment on stuff. If we was just posting "hey, great post" everyday, it would get deleted. He also doesn't leave his name as "buy text books", like a lot of comment spammer do, which is another reason his comments get to stay. I predict a short run for the comment spam buying business.
Read the discussion over at ProBlogger.