It seems Google and AltaVista have had enough of the Keywords spam many sites use to get higher rankings!
I found the 2 following articles that clearly explain why those metatags are now overlooked or have a very small influence on the ranking of your pages :
An End to Metatags (Enough Already, Part 1)
Google Uses Meta Tags Sparingly, But Should You? (Enough Already, Part 2)
So the word of wisdom is : spend time on content, not keywords!