Jeff, and all,
Please see what I think, with all due respect, may be a better tool, called T.A. Toolbar at Grading Essays is Better than Ever. I like it because it's smaller, customizable (thus no need for a database), has a holistic grader mechanism, and a quick plagiary detection system, among other features. Most of the comments that it inserts into a piece of student writing bear hyperlinks to web pages that provide students with more information about the given error. A new version (not yet available) will work with MS Word 2007.