Google prefer "-" instead of "_" As posted on Google support group "We recommend that you use hyphens (-) instead of underscores (_) in your URLs." http://www.google.com/support/webmasters/bin/answer.py?hl=en&answer=76329 Hope this is helps
Google bots looking hyphen as a space and underscore for nothing. we-are-great.html = We Are Great we_are_great.html = wearegreat This was a very long time ago until now and Google still does not fix it. I think Google bots should be smarter by looking underscore as space now.