Wearing a tabard means that your belt is no longer displayed. You can make your belt show up again by unequipping/reequipping it, but it just disappears again soon.
This bug has been around since before TBC, and it’s ridiculous that it hasn’t been addressed. It’s such a small thing that makes anyone wanting to represent their guild look goofy. Just put one guy on this, blue. Come on, this is just SAD.
Come to think of it, this is a great metaphor for the way this game has been supported…