Vista - Questions about binding

How do I bind properties of an unnamed parent? It worked fine in older
builds, now I have to identify the parent in some way. This for instance
wouldn't work:

<Expander Header="Foo" Width="256">
<ListBox Width="{Binding Path=ActualWidth}">
<ListBoxItem>Bar</ListBoxItem>
</ListBox>
</Expander>

The result is that the listbox doesn't fill the width of the expander as
expected.

However if I give the expander a name and drop an ElementName reference into
the mix, it reads out the parent properties. In the past, omitting a
reference to another object would automatically assume the parent. How do I
get this working?

The documentation doesn't help at all here. Which leads me to another
question, why aren't special accessors for RelativeSource et al (for example
/TemplatedParent and /Self) not covered in the documentation? How am I
supposed to find out about things like this?

Thanks.

-mg