There is going to be such functionality rolled out in our upcoming Q3 2010 RadControls for ASP.NET AJAX release.
You will be able to select the TimeSlots of the DayView, WeekView, ResourceGroupedDayView and ResourceGroupedWeekView using the mouse and then using enter to display the inline form for creating appointments. Also it's going to support the TimeSlotContextMenu, i.e. you will be able to create new appointment or new recurring appointment based on the currently selected slots.
Hope you will like this feature!
the Telerik team
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