An issue that has been long overlooked by many web site owners is which characters are recognized by search engines as word separators. I remember having a conversation a few years ago about hyphens vs. plus signs vs. underscores. Underscores have been a no-no because they were not treated as word separators, so after going to all that trouble to create a keyword-rich URL, you were undone by the underscores (e.g., you don’t search for hawaii_vacations, you search for hawaii vacations).
Matt Cutts of Google announced at a recent WordPress conference (WordCamp 2007) that Google is now (or will be shortly) treating underscores in URLs as word separators.
Follow the link above for some other Google SEO tips and insight.