Fons Sonnemans demonstrates a behavior that allows keyboard selection on a ListBox or ComboBox.
Silverlight
doesn't support keyboard selection on a ListBox or Combox. I have
created a small Behavior which fixes this problem. You can attach the
KeyboardSelectionBehavior to a ListBox or ComboBox using Microsoft
Expression Blend. You drag it from the Assets and drop it on your
ComboBox or ListBox. If you have a custom ItemTemplate you will have to
set the SelectionMemberPath property.