File this in the funny bug category. iPhone users in New Zealand who have only recently begun their Daylight Savings Time period are noticing a weird bug on the new iOS 4.1 that seems to be triggering the iPhone alarm an hour ahead of the scheduled time. Apparently, the issue has been plaguing iPhone users (on iOS 4.x) in Indiana, USA since June of this year though it has not been widely noted.
With Australia going into DST early next month and countries like the US scheduled to do the same in November, the proliferation of the bug is likely to increase further in the coming weeks. However, Apple is not likely to issue an update until then. So, expect a proper fix only with iOS 4.2.
Meanwhile, if you want to see an end to this bug, try setting the alarm to non-recurring. It seems to be fixing the bug for the time being.