g1smd - 10:27 am on Feb 22, 2013 (gmt 0)
Underscores visually disappear in underlined links and are indistinguishable from spaces.
Underscores are not fully treated as word separators by search engines.
Spaces and many others should be encoded, and that makes%20the%20URL%20unreadable.
I used to use periods as separators, but I now usually use them only as the separator between filename and extension.