serpsup - 1:54 am on Nov 24, 2012 (gmt 0)
fwiw - I use both underscores and hyphens in most of my urls and they rank just fine. That may sound odd - but I use underscores to separate words, and append a common hyphenated word to the end of many of the urls... so this lead me down the road of both :)
If I had it to do over I'd probably go 100% hyphens though based on this thread. I think underscores don't hurt you as much as some in this thread have said. I have plenty of top 1-3 rankings with underscores and hyphens - and Google and Binghoo can tell the words apart separated by the underscores.