I never did get to the bottom of this bug, but around the time of the start of DST earlier this year something got broken with Calendar Event Alerts. Not sure whether it was DST (since there has been a lot of problems with this) or the iOS update released around that time.
What happened was that if you had a Calendar Event with an Alert set so it was the previous day then the Alert didn't fire on iOS devices.
This seemed to only happen if the Alert time was not one of the preset values on iOS devices but a user-defined value set in iCal on a Mac. e.g. an Event for 08:00 with an Alert 10 hours before (so 22:00 the previous day).
This now all works again under iOS 5
What happened was that if you had a Calendar Event with an Alert set so it was the previous day then the Alert didn't fire on iOS devices.
This seemed to only happen if the Alert time was not one of the preset values on iOS devices but a user-defined value set in iCal on a Mac. e.g. an Event for 08:00 with an Alert 10 hours before (so 22:00 the previous day).
This now all works again under iOS 5