"Stephan Spencer, from CNET's News Blog, posted an informative entry about changes that are taking place with Google's search engine. Of particular interest was the news from Matt Cutts regarding underscores in URLs. Traditionally, underscores were treated differently from hyphens.
Hyphens have long been the preferred method for dividing keywords up in a link. Whereas, URLs with keywords separated by underscores have been ignored by Google � they would only view it as a phrase. However, according to Stephan, that's no longer the case. Matt Cutts stated that keywords separated with underscores are now treated the same as keywords separated with hyphens."
Now this Matt Cutts dude works for Google so that's why i give some credibility that "-" or "_" nowadays are the same, but as you said some time ago the "_" between keywords was in fact viewed by g00gle as a singe word/phrase.