Five kinds of ‘alt’ text is from userfocus.
HTML5: Techniques for providing useful text alternatives at the W3C is an Editor’s Draft. Even though this is a working draft and may change, it is one of the clearest and best written things I’ve seen at the W3C. The editor on this one is Steve Faulkner from The Paciello Group.
WebAIM: Creating Accessible Images looks at alt text from the point of view of scenarios.