iCalender is an IETF standard for adding dates to calender programs such as google calender, Apples's iCal and Microsoft Outlook. An email with an iCalender attachment can appear as a reminder or invitation in the user's productivity suite. This could be added as an option for PREDUE or PREDUEDGST emails, or an additional message could be added for sending iCallender messages when items are checked out. These messages would allow a patron to add item due dates to their own personal calendars on computer or smart phone. ICal.pm is the Perl interface for this standard: http://search.cpan.org/~rbow/Date-ICal-2.678/lib/Date/ICal.pm
This would be nice for the CHECKOUT email too :)
I like the idea.