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.

