Like the Hi Definition format wars, we have an undisputed winner in the to dash or to underscore question.  There is no reason to use an underscore unless a searcher will likely utilize the underscore in the search.  For example, if they were tech heads they might be inclined to search for a bit of code like ‘end_of_file’ for you C junkies out there.  Google will pick that phrase as a single keyword rather than 3 separate keywords in a phrase.

Google will take word1-word2 and read it as word1 and word2 as long as you utilize the dash.  It does not do the same for underscore, so word1_word2 does not read to the algorithm as word1 and word2 but simply word1_word2.

Using our example above a search for ‘page about bagels’ would, all other factors being equal, result in the url somesite.com/page-about-bagels.html ranking higher than the somesite.com/page_about_bagels.html simply because Google is reading the keywords in the former and not the latter.

SO unless you have a specific reason to believe your searcher will search using an underscore as in the ‘end_of_file’ example, stick to the dash.