Basically, it comes down to "follow Google's guidelines to the letter or risk the chance of penalties. The SEO industry has been reporting an increasing amount of manual interventions and websites mistakenly effected by the spammy activity of others.
Coping with SearchEnginePenaltiesSearchenginepenalties are present and pervasive, and are a primary method used by search engines to control webmasters. Here are some penalties that I've seen applied by Google in my own searchengine...
Searchenginepenalties are present and pervasive, and are a primary method used by search engines to control webmasters. Join the Let's talk about searchengine's penalties discussion in the SearchEngine Watch forums.
How WebCrawler Works Site subscribers have access to extensive information about how WebCrawler and Excite work, including submission tips, how the searchengine crawls, spam penalties, how summaries are created and more details.