Page URL - Hyphens or Underscores?

[HMCOE]

What's better for SEO and why? (any references would be greatly appreciated)

For the purpose of a URL of a page is it better to use:

1. Hyphens
2. Underscores

Would hyphens or underscores be better for Google Adwords?

[GoodAtMarketing]

Use Hyphens, people are already accustomed to using them for URLs.

[Unity]

I use hypens so that Google can recognise the keywords.

[David_ODonnell]

It shouldn't make a massive difference, but...

Dashes are typically treated as separators

Underscores are typically joined

Right from the horses mouth: http://www.youtube.com/watch?v=AQcSFsQyct8

[HMCOE]

David, that's exactly what I was looking for! THANKS!