You might have noticed that sometimes the item in the selection box of a combo box disappears when you reopen the drop-down of the combo. For example, in the following image, "Arial" is selected, but doesn't show up in the selection box
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Read the complete post at http://blogs.msdn.com/silverlight_sdk/archive/2008/12/02/where-did-my-combobox-selected-item-go.aspx