iPhoto Matrix Edit is a SIMBL-based plugin for iPhoto that allows you to quickly edit common metadata of photos, especially those commonly used for WebExport galleries.

Download

Warning!!! This is a hack! Please back up your iPhoto Library File. This software interfaces with the deepest depths of iPhoto and as such, is capable of wreaking havoc. That said, I have used it to great effect with no bad things, so it's unlikely. In any event, you have been warned.
First Version: iPhotoTable.tgz

Basic Instructions:

Install SIMBL, put iPhotoTable.bundle into /Library/Application Support/SIMBL/Plugins/, launch iPhoto, see the matrix edit function in iPhoto.

You can also change which fields are displayed, as well as which fields are available for batch edit. You can do this with Interface Builder. Simply open up the main "table.nib" file and add tables accordingly. Table Columns should have display titles, and "identifiers" that correspond to the actual metadata field. Elements in the field drop down the title property for metadata field choice, and the Attributed Title as the display name. This is a little bit techy, bugt this is a total hack that is liable to explode at any time. If you are wondering how I did this, I am thinking of putting together a tutorial for exploring iPhoto and getting similar functionality yourself.