Here is a very helpful Matt Cutts video on hyphens and underscores. http://www.youtube.com/watch?v=AQcSFsQyct8
He confirms that Google see a hyphen as a separator, and does not penalize a domain for having a hyphenated name. I've bought hyphen domains when the .com wasn't available. I've also bought them when it improves the readability or ease of typing by breaking up a 3 key word domain.