Mine never went off again this morning!!!
Deleted all my weekday alarms last night and reset them, woke up this morning 5 mins before they were due to go off, lay in bed waiting and they never went off!
16GB 3GS btw. Pain in the arse.
It seems if you set an alarm for ex, 2mins time from now, it will work, but if you set it to go off "mon/tue/wed/thu" or whatever... it will go off an hour late.