Update: I posted a solution to this problem that I discovered. Let me know if it fixes your problem.
I’ll start by saying that this isn’t the reminder problem that many Windows Mobile users are used to hearing about. (You know, the one where your reminders never fire at all and you miss your appointments?) The problem I am writing about today is irritating on the other end of that scale: my reminders do go off, but I can’t see what they are and have no way to dismiss them. I first noticed this about two months ago, and it was very rare; maybe once per week. Now, it’s happening almost every time a reminder goes off and it’s driving me insane.
If I’m looking at my XV6700 when the reminder fires, I can see that the “toast” — the little notification window that slides up from the bottom of the screen — appears for about one second, then disappears. The reminder bell icon in the title/navigation bar at the top of the screen also disappears. The reminder sound, however, continues to repeat and there’s no way to make it go away other than to soft reset the device. Switching from portrait to landscape mode does not help, either.
I did find reference to another person having the exact same problem. Suggestions were to use MemMaid to clean up bogus or duplicate entires in the notification queue, or to hard reset the device. I don’t have any orphaned or other invalid notifications in the queue, and I am really not thrilled about the idea of having to hard reset my otherwise rock-solid device just to fix this infuriating problem. I started Outlook 2003 on the desktop with the “/cleanreminders” switch to rebuild the reminders, made sure it was synced up with my Exchange account, and then synced the PDA. None of that helped.
I don’t understand why reminders continue to be such a problem for Windows Mobile devices. This is such a basic thing that even $15 electronic organizers get right — reliably — and I just can’t comprehend how devices this advanced can get so very wrong.
I’ll post again if I find a solution to this which doesn’t involve a hard reset, but it’s looking like that’s what I’ll be doing this weekend. I’m in a position to make device recommendations to users and problems like this make it extremely difficult to recommend Windows Mobile devices. Imagine having to tell your boss that the reason he missed his meeting with his boss is because his handheld can’t show reminders reliably.

I haven’t yet done a hard reset. I’m waiting to see if the AKU3 update is actually released in a couple months. Additionally, the frequency of the problem has decreased a bit for some reason. If I can hold off the hassle of a hard reset until then, I plan to do so.
I have this VERY SAME problem.
Did a hard reboot fix it?
I have the VERY SAME problem too!
Matt, I posted a solution to this problem. Give it a shot and let me know if it fixes your problem. I also just added that link at the top of this post.
I read your solution, I hope this fixes this problem for me. My phone has been doing this off and on for several weeks, and driving me nuts! It seems to only do it when I don’t have it set to vibrate-only. I have had time-synchronization turned on for quite a while, but I just turned it off and will hope for the best. I had tried soft-resets to no avail, and have no desire to do another hard reset (had to do one a few weeks after installing Pocket Music) and have to start from scratch.