For my own, I would suggest it is a notion that Google doesn't recognize the _ character. I've just tried some searches myself and the results (one 2,000,000+ results) have pages/sites in the top 10 that use underscores in the name. Try "michigan lodging" (without quotes) for example. 1,600,000+ results and two of the top 10 use underscores.[/reply]
This has been discussed at great length in multiple threads at webmaster world. Having underscores is not a "penalty" it is simply poor SEO over hyphens. If a page is optimized correctly it WILL show in the results. The thing is that underscores hurt, but don't kill your rankings. If the page is SEO tuned very well, the underscore won't do too much damage, but it does not help either. Google most certainly DOES recognize the underscore, it treats it like a caracter, whereas the hyphen is treated as a blank space. There is an employee of google in WW that confirmed this in the many discussions over there on this topic.
Here's the deal, Keyword1_keyword2 is recognized as ONE long word whereas Keyword1-keyword2 is 2 normal words.
Mike