There was a neat utility that worked with iPhoto 5 (and below) called iPhoto Diet. This little AppleScript-based program let you delete all original photos from your iPhoto library (after backing them up, of course!), thus saving you lots of hard drive space. iPhoto diet no longer works for iPhoto 6, but typing the following commands in Terminal does the trick:
$ mkdir ~/.Trash/iPhoto-Originals
$ cd ~/"Pictures/iPhoto Library/Modified/"
$ find . -type f -exec mv "../Originals/{}" ~/.Trash/iPhoto-Originals/ \;
This set of commands creates a folder called iPhoto-Originals in the trash, and moves all of the originals to the trash, while keeping the non-modified originals in place. Just ignore the errors that show up in Terminal; they have to do with the fact that there are some photos in the modified folder (such as movie thumbnails) which don't have corresponding originals.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20060529133344872