The RadComboBox doesn't support client-side binding. Since in your scenario you will be working with small amount of items you could add new items and remove the existing ones using the client-side API as shown on these help articles here
the Telerik team
Register for the Q2 2011 What's New Webinar Week. Mark your calendar for the week starting July 18th and book your seat for a walk through of all the exciting stuff we will ship with the new release!