Thank you for writing.
You are right that the Appointment class has a property Dismissed. The reason is that this class implements IRemindObject interface which is the owner of this property.
Feel free to contact us in case you have further questions.
All the best,
the Telerik team
Q3’10 SP1 of RadControls for WinForms is available for download
; also available is the Q1'11 Roadmap
for Telerik Windows Forms controls.