not2easy - 5:51 pm on Jan 4, 2013 (gmt 0)

from support dot google dot com:

Consider using punctuation in your URLs. The URL http://www.example.com/green-dress.html is much more useful to us than http://www.example.com/greendress.html. We recommend that you use hyphens (-) instead of underscores (_) in your URLs.

Several years ago I read in G support that they consider a hyphen (but not underscores) as a space when looking at links and URLs.