I found this in MacOSXhints, is exactly what I needed! Think I have to share:
Change the modification date on iPhoto imports
Mon, Jan 14 '02 at 05:59AM from: ghopson
I have a bunch of old images I have been trying to load into iPhoto. Now I have spent more time than I should arranging these images into folders based on date, and was dismayed that iPhoto changed that hierarchy when copying the image hierarchy over. Closer inspection of the hierarchy revealed that iPhoto was using the modification date on the image file, and some of the images (either because I had edited the image or rotated it or something) had their modification date changed (duh!), which resulted in iPhoto putting that image in a different folder to what I expected.
I thought about writing a tool to fix the modification dates, but then I came across the shareware tool FileBuddy X (look for it on VersionTracker if link doesn't work).
After downloading and playing with this tool for a while, I found I could open up my images folder containing all my images (I backed up first - be smart!), load the top images folder into FileBuddy, select all, hit "Get Info", change the modification date to the creation date, change all, and then import the images into iPhoto.
Still have to live with the rather terse numbered folders of iPhoto, but at least now my images are arranged with other images taken at the same time, even though I have subsequently edited them.
