heres some discussion about why google “prefers” dashes to underscores:
http://www.mattcutts.com/blog/dashes-vs-underscores/
(matt cutts is a google software enginner, fyi.)
and here is an opposing view:
http://www.simoncox.com/mt3/2006/06/dashes_vs_underscores.php
Thanks to hUNY! for this tidbit…