For creating Google friendly URLs, I was googling for articles and forums to see if hypens(-) is better than underscroe(_).
I found one resource useful from Matt Cutts about the dashes and underscore. Read his article from http://www.mattcutts.com/blog/dashes-vs-underscores
Also there's a supoprt from Google webmaster as well. Google recommends the use of hyphen than the underscore,
Here's the link: