Well, if the iPhone does indeed offer full calendar capabilities in the form of iCal, I may be switching from Verizon to Cingular, despite Cingular’s lack of decent coverage. I am completely hooked on iCal. I can have as many calendars as I want, and view any combination of them at any time. It has a simple but robust to-do list, is sharable, and has an intuitive interface. I rely heavily on my Palm Treo 650 calendar. iCal easily outdoes it.
iCal is compatible (and very similar to) Google Calendar, and will sync with Palm and Windows CE PDAs.