Now, nothing. At least as far as Google is concerned.
Previously, Google had treated dashes and underscores differently in URLs. For example, a URL of yourdomain.com/classic-cars would be read as “classic cars”, while yourdomain.com/classic_cars would be read as “classiccars”.
For most website owners this won't make a big difference, but it does seem to show that Google uses URLs to some degree in their algorithm for ranking sites.
How might this affect you? Well, if you're archiving your newsletters to your site, it would be better to use a keyword rich URL (i.e., /tail-fins.html) as opposed to the date you posted it, (i.e., /20070724.html).
For more on this dash v. underscore change check out Hyphens & Underscores Are Now Treated Equally in Google.com (which, ironically, can be found at http://www.seroundtable.com/archives/014260.html.