I have been cleaning up our intranet portal and realized that the former admin had implemented some content types. We have streamlined how we are gathering information, and also needed to add some new content types. As soon as I started trying to delete the old ones, I quickly realized that there was more going on here than I thought. Specfically, MOSS won't let you delete a content type if a document is using it. That makes sense. So I went through my library and changed all of the documents using the old content type to the new content type. This time when I tried to delete it, same error.