callivert - 1:20 pm on Aug 27, 2007 (gmt 0)

I've never seen anyone offer conclusive evidence that one is more effective than the other with respect to SEO

There is conclusive evidence in the form of a controlled experiment, using the words "callipygian screak quindecillion". [seo-blog.com]
Matt Cutts [mattcutts.com] has also said that dashes are better. (since those articles, Google can now parse underscores in URLs, but they're not the only game in town)