The problem is that when IsEditable = false, RadComboBox contains a ToggleButton, which has its IsTabStop property set to false, which prevents it from getting focus and properly displaying its focused state, which actually is the focused state of the combo box itself. The IsTabStop property has to be false, because otherwise, the button will get focus and will break the keyboard navigation logic of RadComboBox. We are aware of that problem and it will definitely be fixed in one of the next updates of RadControls for Silverlight before the official release in the beginning of November.
As a temporary workaround I would suggest setting IsEditable=true and also IsReadOnly=true. This will slightly change the appearance of RadComboBox, but will "fix" the problem with the focused state.
the Telerik team
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