Until today, Google treated dashes and underscores in the URL differently. A dash (hyphen) between two words in the URL (i.e. keyword-phrase) would tell Google that those are two different words (i.e. keyword +phrase). In the past, an underscore between two words in a URL (i.e. keyword_phrase) would tell Google that those words are a single word (i.e. keywordphrase). Now both underscores and hyphens in the URL are treated as word separators.
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.
Yes, this appears to be a small change but this topic is one of those topics that are constantly coming up in search forums.
Again: dashes (i.e. hyphens) are now equal to underscores where both are now treated as word separators.
Forum discussion at Search Engine Roundtable Forums.