After reading tony's post and the seo books that I have read, search engines are starting to drift away from keywords. They are now looking mainly at the title meta tag and the body content for search engine rankings and relevance. I just did a quick google search and found an article that give one possibly good example of why you would use the keywords meta tag. That reason is mispellings. If your business name is commonly mispelled, put that mispelling in the keywords tag. Obviously, it would be better from strictly an SEO standpoint to include it in your body content, but if you did, that would cause confusion and make you look uneducated!
Here are some more articles I quickly found on Google: