Dan Wahlin has built a derived version of AutoCompleteBox that he calls AutoCompleteComboBox.
Silverlight’s AutoCompleteBox control provides a nice way to filter a list of items as the user types into a TextBox. If you haven’t used it before you can see an example here. In a previous post I showed how you could customize AutoCompleteBox to make it look and feel more like an editable ComboBox so that users can click a down arrow to see the list of items and also filter the items by typing into the TextBox.