I just finished up work on an iPhoto plugin that allows you to explicitly change various metadata fields in a spreadsheet format. It is a SIMBL-based plugin, and is available for download:
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.