Hyphens are better than underscores, and we intentionally used hyphens instead. The main reason is readability. If I have a link like my_home_page and then style it on my site (or another website) such that links are underlined, then I can no longer tell if it is spaces or underlines in the word.

From my experience, hyphens provide very good SEO; you can tell that with our own silverstripe.com in terms of how effective we rank for "open source cms", etc. Secondly, using the actual <title> element reinforces your name is very good for SEO.

Check out Jill Whalen at http://www.highrankings.com/ for more of this sort of info

Hyphens are better than underscores - shearch engines in general see "open_source_cms" as one word, where as they distinguish the distinct words in "open-source-cms" becuase hyphens are used.

Personally, i tend to use underscores as a force of habit when naming files and images (and ids, classes, function names etc etc), but I really need to get into the habit of using hyphens becuase technically it's a better method.