There has been much debate on this topic, for probably FAR too long…should you use underscores or hyphens within your URLs?
I spent years defending the underscore, because I had seen sites do well on Google, but at the same time I had wondered if it was as “friendly” to other search engines. Recently, I have pushed forward on the use of hyphens and I’ll tell you why.
Our friend Matt Cutt’s (Google Search Engineer) uses hyphens within his blog, located at www.mattcutts.com/blog . Here’s an example: http://www.mattcutts.com/blog/bot-obedience-herding-googlebot/ (that’s a pretty good article to read, by the way).
So, if it’s good enough for Matt Cutts, it’s good enough for me. Seems he would know Google pretty well. End of discussion.