Lawrence,
iPhoto should organize the photos based on capture date of the image. This is true unless the files have been edited prior to importing into iPhoto. For some reason iPhoto will then use the last modified date of the file. Using the calendar in iPhoto you can drill down to a specific date and it should show just those photos in the main view pane. That being said, iPhoto will put all of the images from a particular import session into a single "roll" regardless of capture date. However, that should not impact the calendar function. You can also make new rolls of film from specific photos if you want to break them into more rolls.
Hope this helped!
Sam
