It’s one of the many rankingfactors within Google: your urls. One of the talks I have a lot is whether people should use dashes or underscores in their urls because many sites have underscores. The answer is dashes off course. But why? Matt Cutts explains it in this video. Over at Searchengineland Matt McGee extracts the differences in behavior when it comes to dashes and underscores between Google and Bing.