This may be stating the obvious, but I think the original date and other info for each photograph is stored in the library.data, cache.data and dir.data files in the original iPhoto Library folder. Did you back up those files with your images and replace them in exactly the same location as before? If you did I agree with you that what you did should work fine - in which case I'm stumped too.
I am experiencing problems related to yours when I attempt to re-locate my iPhoto images folder to a hard-drive other than the one OSX is installed on - or any other folder for that matter. I may be missing something here, but I have not been able to find a way to specify a new path for images and library data files. I was hoping iPhoto could simply rebuild its library based on the original library files and a new path - or that I could at least make it work by editing the library files manually, but I have not been able to make this work.
The AlbumData.xml file in the iPhoto library folder contains these tags at the start:
<key>Archive Path</key>
<string>/Users/username/Pictures/iPhoto Library</string> - which seems to indicate that the path is not intended to be hard-coded in the iPhoto application. I guess the .data files must contain something similar, but if they do I have not been able to find it. So - no luck.
Anyone successfully backed up and then restored their iPhoto Library, or moved their Library relative to its original location? Anyone know anything about this?