According to this CNET article, Google is going to treat underscores as word separators (No s**t, Einstein).

One key development that Matt shared with the audience was that underscores in URLs are now (or at least very soon to be) treated as word separators by Google. That’s great news, because it historically hasn’t been that way. Back in 2005, Matt stated that Google did not view underscores in URLs as word separators. That meant that in a URL like http://www.mysite.com/iphone_review.html Googlebot couldn’t “see” the words iphone or review. Instead it read iphone_review as one word.

Read the entire article here.