How can I overwrite my ListBox's ItemTemplate and still keep the DisplayMemberPath?

Why exactly do you want to keep DisplayMemberPath? Its just a shortcut for an ItemTemplate containing a TextBlock showing the value in DisplayMemberPath. With your own ItemTemplate you have much more flexibility what and how you want to display.

Just add a TextBlock into your ItemTemplate and set Text="{Binding Value}" and you have what you want.

As described here:

This property is a simple way to define a default template that describes how to display the data objects.

DisplayMemberPath provides a simple way to a template, but you want a different one. You can't and you don't need both.

I still can't figure out how to get it to draw using the DisplayMemberPath, however I did find the piece I was missing to get it drawing using the ItemTemplate - I needed the ContentTemplate binding

<RadioButton 
    Content="{TemplateBinding ContentPresenter.Content}" 
    ContentTemplate="{TemplateBinding ContentPresenter.ContentTemplate}"
    IsChecked="{Binding Path=IsSelected,RelativeSource={
                        RelativeSource TemplatedParent},Mode=TwoWay}" />

Then in my XAML I can write:

<ListBox Style="{StaticResource RadioButtonListBoxStyle}"
         ItemsSource="{Binding MyCollection}"
         SelectedValuePath="Key">

    <ListBox.ItemTemplate>
        <DataTemplate>
            <TextBlock Text="{Binding Value}" />
        </DataTemplate>
    </ListBox.ItemTemplate>
</ListBox>

Thanks to dowhilefor for pointing out that DisplayMemberPath is simply a shortcut way of writing the ItemTemplate, and that both cannot be set at once.

I have been researching this same problem since morning and this thread helped me a lot. I investigated the solution and found out there is a way to support DisplayMemberPath even after defining a custom ListboxItem controltemplate in a style . The key is adding

    ContentTemplateSelector="{TemplateBinding ContentControl.ContentTemplateSelector}"

to the contentpresenter( the radiobutton in case of this thread).

The reason this will work is because internally the displaymemberpath property causes the Listbox to assign the ContentTemplateSelector to a "DisplayMemberTemplateSelector" template selector. Without this TemplateBinding in place, this selector does not take effect.

Cheers!