Matt Cutts of Google has said that url-like-this is better because that format makes Google see 3 words (url, like, this). While url_like_this makes Google see one word (url_like_this). He has hinted that they might change it in the future. That said, I've had good results getting to search engine position with urls_like_this.
the url-like-this.html is also better for users as they dont have to press the shift key when typing in the URL
I believe like freelistfool said, Matt Cutts has stated that - is better than _ for URLs. The - is viewed as a space and sometimes the _ is viewed as part of the text. The reason why _ is viewed as part of the text is because a long while back it was used as part of variables and/or constants in programming.
I attended Bruce Clay's SEO toolset course and he said that URLs with dashes are considered more "spammy". Can anyone confirm this?
From a visitor standpoint - is better then _, as - requires the use of the shift key. http://www.mattcutts.com/blog/dashes-vs-underscores/
It does not matter If you have a real SEO at work... the battle is over before the search engines need to worry about - or _
As earlier posters have indicated, I also agree "-" is better than "_" Of course, you can achieve top page rank with the right SEO methods regardless. However, that process requires taking every advantage and employing every optimization method. Since Cutts has stated that the "-" has an adventage; why go with the _ if it's only a disadvantage? Take care and best of luck.
There is no difference between the two SEO-wise, though I prefer the dashes for accessibility and usability reasons. ...and... There is no difference, and it was Matt Cutts who broke the announcement that the two would be treated equally. Misinformation. They can however make it harder for people to remember the URL if they're overdone. No advantage for robots; the advantage is for people. (Is that an underscore or a blank space? I can't remember...)