it would be really nice if you could select a timeslot on the calendar and create a new activity with the correct start and stop times like in Google Calendar. Here is how I would see this working...
Select the timeslot on the calendar and a new activity screen would pop up with the due date start and due date end already populated. From here you would select your contact and the rest of the activity details. Saved would take you back to the calendar and refresh the calendar.