Krename is able to do file renaming based on mp3 tags. When emerging Krename there is no USE flag to set this option. So you have to manually emerge kdemultimedia-kfile-plugins. I think a "mp3" use flag should be added in orther to automatically emerge the needed ebuild to get mp3 tag support!
These are plugins, not real dependencies. I've added a post install message to the most recent ebuild.