Everybody has come up with this theory that leaving links through your comments on DoFollow blogs actually helps out your SEO. Google is such an intelligent search engine so how do you know whether Google actually counts links from blog comments toward your SEO? Most blogs use either Blogger or Wordpress so if Google wanted to they could easily identify whether the link from a blog is from a comment or was placed by the blog owner themselves.
It's a good question and I've thought about this a lot. Rather like forums, I think Google treats a blog like any other web page, though you'd think they were clever enough to know the difference. Google are against user generated spam, but the only way that they deal with it is with nofollow and if a blog is dofollow, there's no concrete reason to think that the link won't carry as much value as any other. I'd be interested to hear other's thoughts.