http://www.engadget.com/2010/12/31/psa-iphone-alarms-not-working-come-new-years-day-2011/
Most advanced mobile operating system iirc
Update: 9to5 Mac says that the bug will get fixed all by itself once the calendar hits January 3rd. We haven't confirmed this, but if it turns out to be the case, that's pretty good news. In less good news, the issue seems to be affecting iOS 4.1 and 4.0.2 as well.
A one off alarm or recurring alarm?