An alternate version of another gist ( https://gist.github.com/2363747 ) but to be used with PowerShell's Select-Xml instead

Select-NamespacedXml.ps1

<#
.EXAMPLE
Select-NamespacedXml -XPath //def:SomeElement -XmlDocument $XmlDoc -DefaultNamespacePrefix def

.EXAMPLE
Select-NamespacedXml -XPath //foo:AFooElement -XmlDocument $XmlDoc

Assuming the document looks like this:
    <Root xmlns="http://namespaces/vanilla" xmlns:foo="http://namespaces/foo">
      <foo:AFooElement>Hello</foo:AFooElement>
      <SomeElement>Bye</SomeElement>
    </Root>

Achieving the same results with PowerShell's default Select-Xml would require this:

    Select-Xml -XPath //foo:AFooElement -Xml $XmlDoc -Namespace @{ foo = 'http://namespaces/foo' }

#>
[CmdletBinding()]
param (
    [Parameter(Mandatory=$true, Position=0)]
    [string]
    $XPath,

    [Parameter(Mandatory=$true, Position=1)]
    [xml]
    $XmlDocument,

    [string]
    $DefaultNamespacePrefix
)

$Namespace = @{}

$DefaultNamespace = $XmlDocument.DocumentElement.GetAttribute('xmlns')
if ($DefaultNamespace -and $DefaultNamespacePrefix) {
    $Namespace.Add($DefaultNamespacePrefix, $DefaultNamespace)
}

$XmlDocument.DocumentElement.Attributes | 
    Where-Object { $_.Prefix -eq 'xmlns' } |
    ForEach-Object {
        $Namespace.Add($_.LocalName, $_.Value)
    }

Select-Xml -XPath $XPath -Xml $XmlDocument -Namespace $Namespace