Calendar wise, I really hope I will be able to accept calendar invites from the iPhone. Thats something I have really hated about the Treo. I don't always have time to sync up, and its very annoying when out and about seeing a new calendar invite come through but not even be able to see what it is, or when it is.
This drives me and my co-workers nuts, Steve if your listening ... you could get all of them to buy it if it had calendar invite support directly on the phone
(I am purchasing one regardless, but will be annoyed if I can't accept invites)