http://www.cnn.com/2010/TECH/mobile/11/05/apple.alarm.daylight.savings/index.html?hpt=T2

From the article:
Apple iPhone users in the United States must also remember to delete and then reset their phone's alarm clock -- otherwise they may be an hour late for work on Monday morning.
A glitch in the iPhone's operating system will cause recurring weekday alarms not to ring on time on Monday morning because of the end of Daylight Saving Time, which occurs at 2 a.m. on Sunday in the United States.