Matt Cutts on SEO

...And speaking of putting a dash in URLs, hyphens are often better than underscores. african-elephants.html is seen as two words: "African" and "elephants". african_elephants is seen as one word: african_elephant. It’s doubtful many people will be searching for that...

http://www.mattcutts.com/blog/ guest-post-vanessa-fox-on- organic-site-review-session/