For a long time now, people have been saying that dashes are counted as spaces in a URL, and underscores do not. What I'm curious about is why anyone cares. Well, people optimizing for MSN might, but MSN doesn't really bring such a significant amount of traffic anyway. Background: MSN seems to care a little too much about keywords in a URL.

Notably, I've wondered for awhile if it almost makes sense to embrace the "_" as a sign of solidarity to an ascii character that seems to be getting neglected and beaten up for no reason at all. That, and it seems to me that this whole debate has been causing spammers to beat the "-" to death, which may be a bad thing if users get conditioned to see dashes as a loose indicator for spam.

Personally, I think if search engines actually regarded the dash vs. underscore issue as important, they would just fix the issue. It wouldn't really upset much, as the underscore really doesn't get much action on the internet, except in the URL. This just underscores (ha ha -- bad pun) the fact that nobody cares!

I still use the regular dash, "-," in URLs, as I always have, but I see plenty of major sites using "_" even after redesigns that restructured their URLs. Personally, I really don't think it matters. In my mind the purpose of putting keywords in the URL is to make the URL look relevant and jump out at you. It's a more effective use of real estate in your SERP.

http://www.lawyerseek.com/Practice/Pharmaceutical-Injury-C1/

... looks better than

http://www.lawyerseek.com/Practice/-C1/

... but

http://www.lawyerseek.com/Practice/Pharmaceutical_Injury_C1/

... doesn't look any worse to me either. There are better things to worry about.