I've tried your solution and got an exception when adding new appointment. It happens between AppointmentCreating and AppointmentCreated events.
The value "New Subject : 18.11.2010 0:00:00 - 25.11.2010 0:00:00" is not of type "MyWpfApplication.MyAppointment" and cannot be used in this generic collection.
at System.ThrowHelper.ThrowWrongValueTypeArgumentException(Object value, Type targetType)
at System.Collections.ObjectModel.Collection`1.System.Collections.IList.Add(Object value)
at Telerik.Windows.Controls.ScheduleView.ScheduleViewDataConnection.HackCollectionViewBug(Boolean isCommit) in c:\Builds\WPF_Scrum\Release_WPF_2010_Q3\Sources\Development\Controls\ScheduleView\ScheduleView\Controls\ScheduleViewDataConnection.cs:line 887
is my custom appointment class. By the way, e.Appointment is of correct type MyAppointment on AppointmentCreating.
Could you please advise?