NoFollow and DoFollow Blog - Comment Links Affect Search Engine Optimization
Many sellers of information products leave comments on blogs, especially popular ones with high Google PageRank, thinking that they are getting those good inbound links to their sites. But there's a problem. Most blogs put a "No Follow" tag in the link to your website. Sure, readers can click on it and check you out, and that's a good thing. But you get no SEO benefit.