The pattern is:
public string Text {get;set;}
and the binding is
{Binding Text, RelativeSource={RelativeSource FindAncestor, AncestorType=Window}}
If you want the binding to update automatically you should make it a DependencyProperty.
I think 3.5 added ElementName
to bindings, so the following is a little easier:
<Window x:Name="Derp" ...
<TextBlock Text="{Binding Text, ElementName=Derp}"/>
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