Did you ever find a solution to this? I have found the same sort of issue with the Task reminders on my Blackberry. It is terribly frustrating, as I am a heavy task user and everyday a few of them do not go off. I like everything about the phone except this one issue, which is getting to be enough of a problem that I am considering trying to get rid of the phone, except that I've had it for longer that 30 days.
I find it very hard to believe that RIM would not correct a problem as glaring as one of the core PIM functionalities simply not working.