After upgrading to WordPress 2.7 “Coltrane,” trackbacks and pingbacks to other websites and my own stopped working. Deactivating the plugins did not work either. I took the risk to downgrade to Version 2.6.5 after making a backup of the core files. I uploaded Version 2.6.5 files allowing the FTP program to overwrite the existing Version 2.7 files except the wp-content folder. After the all files have been uploaded, I ran the upgrade script, downgrade in this case. Everything seems to be working right for now. Most importantly, trackbacks and pingbacks are working again.