Its official now. What most SEO professionals did know for yours is now officially stated on there Webmasterscentral Blog: "Google does not use keywords meta tag for ranking ". And what about description tags?
"Q: Does Google ever use the keywords meta tag in its web search ranking?A: In a word, no. Google does sell a Google Search Appliance, and that product has the ability to match meta tags, which could include the keywords meta tag. But that's an enterprise search appliance that is completely separate from our main web search. Our web search (the well-known search at Google.com that hundreds of millions of people use each day) disregards keyword metatags completely. They simply don't have any effect in our search ranking at present."
Unofficially: description tag for ranking?
"Q: Does this mean that Google ignores all meta tags?A: No, Google does support several other meta tags. This meta tags page documents more info on several meta tags that we do use. For example, we do sometimes use the "description" meta tag as the text for our search results snippets, as this screenshot shows:Even though we sometimes use the description meta tag for the snippets we show, we still don't use the description meta tag in our ranking."