Xml Binding - Why does XPath return nothing?

I am trying to bind to an XmlElement but it isn't working. Why isn't the
third binding returning any text? These are my Xaml fragments:

<TextBlock Text="{Binding Path=OuterXml}"></TextBlock>
<TextBlock Text="{Binding XPath=*}"></TextBlock>
<TextBlock Text="{Binding XPath=FirstName}"></TextBlock>

This is the text returned from the first binding:

<Person xmlns="jdata">
<PersonID>4</PersonID>
<FirstName>Justin</FirstName>
<LastName>Magaram</LastName>
</Person>

The second binding returns '4' which I think is correct. But the third
binding returns nothing. I've tried different flavors:

../FirstName
jdata:FirstName

The XmlDataProvider is created in code. There is no reference to the
XmlDataProvider in the markup. The Xml looks like this:

<Sample xmlns="jdata">
<Person>...
<Person>...

I load it into a DataSet:

sampleDataSet.ReadXml(@"Sample.xml");

I then create an XmlDataDocument:

xmlDoc = new XmlDataDocument(sampleDataSet);

To do any XPath against xmlDoc, it seems I need to create a namespace manager:

namespaceMgr = new XmlNamespaceManager(xmlDoc.NameTable);
namespaceMgr.AddNamespace("default", sampleDataSet.Namespace);

I then create my XmlDataProvider:

XmlDataProvider xp = new XmlDataProvider();
xp.Document = f.XmlDoc;
xp.XmlNamespaceManager = f.NamespaceManager;
xp.XPath = "/";

Any here I assign my listbox to the data:

XmlNodeList nodeList =
xp.Document.SelectNodes(@"/default:Sample/default:Person",namespaceMgr);
listbox1.DataContext = xp;
listbox1.ItemsSource = nodeList;

By the way, it is really annoying to have to preface all elements in the
XPath with 'default:'. Is there any way to avoid this?