According to numerous complaints on twitter, it seems that the iPhone's alarm clock will not function properly come New Year's Day. This particular glitch in the iOS software will affect all users who use the alarm clock without recurring alarms. According to 9to5mac, the glitch will fix itself on January 3rd. You may recall a similar bug in the iOS software which was later on fixed in iOS 4.2.1.
For now, make sure to set all your alarm clock to reoccur, or repeat on the days you want it to. This bug is present in 4.2.1 and earlier versions of iOS 4.
Update: Apple has responded to this issue
We're aware of an issue related to non repeating alarms set for January 1 or 2. Customers can set recurring alarms for those dates and all alarms will work properly beginning January 3.