Matt Cutts rekommenderar bindestreck (dashes):
"... some people thought that underscores are the same as dashes to Google now, and I didn’t quite say that in the talk. I said that we had someone looking at that now. So I wouldn’t consider it a completely done deal at this point. But note that I also said if you’d already made your site with underscores, it probably wasn’t worth trying to migrate all your urls over to dashes. If you’re starting fresh, I’d still pick dashes."
Whitehat SEO tips for bloggers
och som tidigare nämnts:
Dashes vs. underscores
(Edit: la till en länk)