MarketingProfs.com has a good article called Beware of Black Hat SEO:

Search engine spammers never prosper. Sooner or later, they get caught. And when they do, it’s almost never pretty.

Consequences can include ranking penalties, removal of the site’s “voting” power (i.e., ability to pass PageRank), incomplete indexation (i.e., a partial site ban), or, worst of all, getting “graybarred” (i.e., a total site ban, when the PageRank meter in the Google Toolbar is grayed out).

One site they talk about as an example is what happened to BMW.de:

BMW recently had its entire BMW.de site banned from Google for a period of time because it created “doorway pages”—pages full of keyword-rich copy created solely for the search engine spiders and never for human viewing.

So what are some example of “Black Hat SEO” that can get you banned or punished by Google? They have a nice list in the article, but a few that I’ve actually seen recently would include:

• Hidden or tiny text—making the text the same color as the background; or shrinking the font size way down; or employing noscript, noframes, iframes, or hidden
  tags to hide text and/or links
• Keyword stuffing—the excessive placement of keywords within web pages (e.g., in alt tags, meta tags, etc.)
• Targeting irrelevant keywords—optimizing for popular keywords that have no relevance to your business
• Blog comment spamming—posting bogus comments to blogs, with links to your site
• Splogging—creating blogs and posting to them content stolen from other sites

So how do you make sure your SEO is “white hat”? Keep this in mind: your target audience is the reader. Design for people, not Google. You can optimize what you’ve designed for people so that Google understands it also.

Rule of thumb: if you do a design tactic that is for the search engine only, it’s probably spam.