The best list of functional points I can imagine for the Tasks/Calendar applications is the PalmOS application DateBk6 (http://www.pimlicosoftware.com/datebk6.htm) from Pimlico Software. The screenshots have gotten a bit gaudier since I first used it :-), but I have never seen anything on a PDA approaching its functionality.

If I had to pick a single thing, I would say that implementing a quick way of taking a task and automatically generating a linked calendar item is most important. In the N900 metaphor, that would be best accomplished by either a drag and drop into a day in the calendar--or via a long-hold action that would than generate a "pick date" dialog.