Lets put this to rest - here's exactly how Google looks at dashs vs underscores - written by their rep:
http://www.mattcutts.com/blog/dashes-vs-underscores/
Note that they treat word1_word2 differently than word1-word2
in order for a search to show for the first example the searcher would have to type in the whole phrase wheras the second example Google treats as two separate words