This is a compilation of stuff Matt Cutts has said historically, minus some of the more recent stuff here, here, and here. I decided I’d dig backwards and document some of the older stuff. I dated it accordingly. Here it is:
Matt recommends using dashes over underscores to delimit words in urls. 2005.
Google does not algorithmically penalize for dashes in the url despite the fact that some have raised it as a possible heuristic for spam detection. I think WordPress pretty much precludes this anyway. 2005.
Google updates their index data