Google Team released recently a video that clarifies an issue of Underscores vs. dashes in URLs and its impact on SEO.
Matt Cutts from Google explains the difference between how Google interprets underscores and dashes in a URL, and tells a history of this issue. The main conclusion of this message is that if you create a new website you should structure your URLs with dashes, however the issue does not have a huge impact on SEO, so if your existing site uses underscores, you should not worry much and keep it the way it is.
More: Underscores vs. dashes URLs (news, Wednesday July 14, 2010)
