From Grant Erickson:
Metadata Export for iPhoto is an export plugin for iPhoto 7 which allows you to export metadata about your iPhoto library photos, including:
Comments - comments about the photo.
Archival Date - Archival date of the photo.
Creation Date - Creation date of the photo.
Modification Date - Modification date of the photo.
Format - the file/image format of the photo.
Height - height of the photo, in pixels.
Keywords - keywords assigned to the photo.
Path - absolute path to the photo.
Rating - ranking/rating of the photo.
Rotation - rotation of the photo, in degrees.
Title - title (name) of the photo.
Width - width of the photo, in pixels.
to a number of different file formats, including:
Comma-separated Values (CSV)
Extensible HTML (XHTML)
XML Property List
With the exported metadata, you can do things like generate and print labels for organizing hard copies of your photos, slides or negatives. Or, you can import the data into a database program and run and generate queries or reports to gain insights about your iPhoto library.