Ive read in the conspiracy forums that if you use an underscore google treats it as one word and hyphens as 2. However, http://www.google.com/search?q=casinobonus&ie=utf-8&oe=utf-8&aq=t&rls=org.mozilla:en-US:official&client=firefox-a
you will notice both casinobonus and casino bonus is highlighted which tells me google can still pick out the keywords eventhough I did not use a hyphen. So bottom line, underscore, hyphen or running the words together, I don’t think it really makes a difference with the exception of it looking a little prettier with a hyphen.