One of the things that makes Datebook on the Palm work is that Palm is a realtime OS. It doesn't multitask.
It always knows what the current state of everything is. When you open the calender, it knows you want to look at now, today.
Maemo's not. If your PIM's not running, you get no alarms. It won't wake up if it's off. Palms don't have an off.
Nokia has said all along that the NITs are not PDAs. They're right.