if an user creates an Appointment marking it as private, the Windows Phone 7.5 standard Calendar app shows it as usual, with the flag 'personal' (and not 'private', not in the Italian translation at least) checked.

but a thirdy app cannot show it properly, because if Appointment.IsPrivate = true, then Appointment.Subject and Appointment.Details are null.

in my opinion, the 'personal' - or 'private' - flag is meant to control Appointment visibility to shared calendars with other users - think at my shared calendar with my wife and an Appointment to buy her a gift. infact, if I create a Google Calendar Appointment and mark it as 'private', it will be synchronized with my Windows Phone Calendar. then, it's up - it would be up - to the Windows Phone Calendar not to include it in any sharing feature with other users.

that's why I think Appointment.IsPrivate = true behaviour should be handled by the thirdy party app, leaving all the properties visibility enabled.