Big news for small punctuation! SERoundtable.com reports that underscores will now be treated as word separators as hyphens have in the past. This might mean a little ranking change up (maybe we already saw this change in effect, or it could be occurring right now). Most wordpress blogs (and others) already use hyphens in the URL naming structure, but I know many sites use exclusively underscores out of habit (I used to!).
Matt Cutts on dashes vs underscores
The announcement from WordCamp (covered by News.com)
Let me point out that domain names only allow hyphens, not underscores, and I don’t think they will ever implement underscores in domain names.
Some links about using your URLs to enhance your SEO:
Keywords in URL the New Google Search Optimization Winner?