Many people believe that you can use an underscore to separate_words_in_an_url, and that Google will recognize the words and count them for ranking purposes. I have read many debates about this: some people believe that Google indeed counts underscores as word separators, while other webmasters claim that you must use a hyphen or other character for Google to recognize the words as distinct. GoogleGuy has hinted in the past that hyphens are preferred. Recently, this debate reheated when Google made a change to it's "search word highlighting" feature in the search results: suddenly, keywords separated by underscores were being displayed in bold. The question then became whether the change was part of the ranking algorithm, or just the display mechanism.
Today, GoogleGuy finally confirmed this without any shadow of a doubt: "If you use an underscore '_' character, then Google will combine the two words on either side into one word. So bla.com/kw1_kw2.html wouldn't show up by itself for kw1 or kw2. You'd have to search for kw1_kw2 as a query term to bring up that page." [Link to Quote] I think we can put that debate to bed finally, at least for now.