I am using the Attachments plugin, but this question I'm sure is applicable to the regular media library. What I want to do is have an 'attachment' (metadata, title, caption) but with no image associated with it. Reason being, I want to treat attachments as 'project items' which can either be an image or a block of text. Is this possible?
Then you should use custom fields, attachments are for something to attach from your media library. Custom fields on the other hand can be anything you want.
For a plugin I recommend , http://wordpress.org/extend/plugins/custom-field-template/