Maybe this is getting too deep into a philosophical discussion on calendars and tasks but for me, anything I put into the calendar represents a hardstop from whatever I am doing unless I just choose to skip it and/or reschedule.

I wonder if what you are really looking for is a simple reminder/alarm app. On the Treo, I used BugMe which was nothing more than an alarm-able notepad, very useful. For reminders of untimed activities, I use ideaMatrix on the Blackberry and set an alert to a note. Once the alert goes off, you can choose right away when the next alert should go off. It's excellent for those shopping list reminders so I can snooze it for another time when I know I'll be at the market for sure. Hope this helps.

-h