One of the things mentioned at the SXSW “How To Trick-out Your Blog”:http://2005.sxsw.com/interactive/conference/panels/?action=show&id=IAP0060 session was that Google seems to like dashes a bit more than underscores in URLs.
I’ve since found some discussion that seems to confirm this:
* “Mark Carey”:http://www.markcarey.com/googleguy-says/archives/discuss-underscores-are-not-word-seperators-in-google.html
* “Phil Ringnalda”:http://philringnalda.com/blog/2004/04/underscores_are_bad_mmkay.php
* “WebPro News”:http://www.webpronews.com/ebusiness/seo/wpn-4-20040802DomainNameDilemmaDoDashesorUnderscoresGooseGoogleRankingsMore.html
I seriously doubt that the impact of the switching from underscores to dashes is significant enough to warrant changing current URLs (and generating a sea of broken links that kind of defeat the purpose), but for new sites it makes sense to use dashes instead of underscores.
And as an added bonus, the URLs are also more human-friendly as underscores are harder to discern when they are linked (the underscore blends into the underlining of the link).