I just downgraded WordPress 2.8 to WordPress 2.6.5. The same problem with trackback and pingback not working cropped up just like after WordPress 2.7 was installed. When I discovered that WordPress 2.8 was not sending trackbacks and pingbacks, I deactivated all plugins and used a different template. It was still no go. I deleted wp-admin and wp-includes, overwrote files in the root folder with similar files from WordPress 2.6.5 and copied the two folders over. As soon as I ran the upgrade script for WordPress 2.6.5 trackback and pingback from published entries were sent out. Looks like I am stuck with WordPress 2.6.5 for now until I find a way to make trackbacks and pingbacks work for the newer versions.