Back in September 2007 Matt Cutts posted about how Google were going to be removing the difference between underscores and dashes as word separators.
Last year, there were several posts indicating that this wasn’t yet implemented. However with maybe 200 different elements to the algorithm, I think that it is perhaps not quite this simple. Google certainly manages to highlight the right words in the URLs; it certainly should be able to do so when evaluating it.
Looking at the results from searches today, I would say that Google certainly treats them differently if used in the search term. E.g. a search for [seo blog] is significantly different to [seo_blog] – this was their original intention when coming up with the variance. Interestingly [seo-blog] comes up with a different set of results again, so there is a ranking difference here too.
So, although the matter isn’t closed, I would still recommend using hyphens rather than underscores (they are better for users anyway). If you are writing a new site, then dashes are the way to go. If you have an old site using underscores that ranks well, this is not likely to be holding the site back in a significant way, so there is no reason to change it.
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