Hello.
I'm using silverlight 4 and I solved it like that:

<ComboBox x:Name="COMBO_NAME" Grid.Column="2" Grid.Row="3" Margin="2" ItemsSource="{Binding ITEM_SOURCE}" SelectedValuePath="SELECTED_VALUE" SelectedValue="{Binding OBJECT_IN_ITEM_SOURCE, Mode=TwoWay}" MinWidth="50" Style="{StaticResource styleComboBoxObscuro}" ItemContainerStyle="{StaticResource styleComboBoxItemObscuro}">
<ComboBox.ItemTemplate>
<DataTemplate>
<TextBlock Text="{Binding TO_SHOW_IN_ITEM}">
<ToolTipService.ToolTip>
<TextBlock Text="{Binding TO_SHOW_IN_TOOLTIP}"/>
</ToolTipService.ToolTip>
</TextBlock>
</DataTemplate>
</ComboBox.ItemTemplate>
</ComboBox>

hope be helpfull!