The rolls in the iPhoto library are invariably sorted by the import date of the roll. If rolls are imported in a different order than they were actually taken (for example, migrating a photo archive to iPhoto), the chronological order of the rolls is lost and cannot be restored.
Or so I thought, until I noticed that the field of the info dialog which displays the import date and time actually is an input field. Its value can be changed! And the roll is rearranged in the library according the changed value.
To display the info dialog for a roll: view library by film roll (toggle by Shift-Command-F), select a roll, display the info dialog (toggle by the i button under the source list). The Date: field is the one. Although the value contains date and time, it is possible to just enter the date; in this case, the time (strangely) is taken from the previous value.
On a side note: A curious bug shows up for rolls that contain only one photo. If such a roll is selected, then the info dialog of the picture is shown instead of the one of the roll, thus the import date of the roll is inaccessible. Duplicating the picture solves the problem. To duplicate: open roll, select picture if not yet selected, duplicate by Command-D.
This hint applies to iPhoto 4.0.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20040304060727160