New-WebServiceProxy : The request failed with HTTP status 401: Unauthorized.
At C:ScriptsGetItemID_UsingWebServices.ps1:1 char:26
+ $WS = New-WebServiceProxy <<<<  -uri http://server/sites/promo/_vti_bin/SiteData.asmx
    + CategoryInfo          : ObjectNotFound: (http://server...n/SiteData.asmx:Uri) [New-WebServiceProxy], WebExcept
   ion
    + FullyQualifiedErrorId : WebException,Microsoft.PowerShell.Commands.NewWebServiceProxy
Property 'Credentials' cannot be found on this object; make sure it exists and is settable.
At C:ScriptsGetItemID_UsingWebServices.ps1:2 char:5
+ $WS. <<<< Credentials = [System.Net.CredentialCache]::DefaultCredentials
    + CategoryInfo          : InvalidOperation: (Credentials:String) [], RuntimeException
    + FullyQualifiedErrorId : PropertyNotFound

You cannot call a method on a null-valued expression.
At C:ScriptsGetItemID_UsingWebServices.ps1:12 char:26
+ $GUS = $WS.GetURLSegments <<<< ($ItemURL, $webID, $bucketID, $strListID, $strItemID)
    + CategoryInfo          : InvalidOperation: (GetURLSegments:String) [], RuntimeException
    + FullyQualifiedErrorId : InvokeMethodOnNull

$WS = New-WebServiceProxy -uri http://server/_vti_bin/SiteData.asmx
$WS.Credentials = [System.Net.CredentialCache]::DefaultCredentials
#$WS.Credentials = [System.Net.NetworkCredential]::("user", (ConvertTo-SecureString "password" -AsPlainText -force), "domain")
$ItemURL = "http://server/sites/promo/sales/testLib/TestFile.txt"
$GUS = $WS.GetURLSegments($ItemURL, $webID, $bID, $strListID, $strItemID)
Write-Host $strItemID

$webID = $null
$bID = $null
$strListID = $null
$strItemID = $null
$GUS
$uri = "http://Server/sites/Promo/TestDocLibrary44/_vti_bin/SiteData.asmx"
$WS = New-WebServiceProxy -uri $uri -UseDefaultCredential
$ItemURL = "http://Server/sites/Promot/TestDocLibrary44/2011102100006b.pdf"
$GUS = $WS.GetURLSegments($ItemURL, [REF]$webID,  [REF]$bID,  [REF]$strListID,  [REF]$strItemID)
write-Host $strItemID.value # returning null or blank
write-Host $webID # returning null or blank
Write-Host "Test "

$WS.Credentials = [System.Net.NetworkCredential]::("USERNAME", (ConvertTo-SecureString "PASSWORD" -AsPlainText -force))

$WS.Credentials = [System.Net.NetworkCredential]::("USERNAME", (ConvertTo-SecureString "PASSWORD" -AsPlainText -force), "DOMAIN")

$WS = New-WebServiceProxy -uri http://server/sites/promo/sales/_vti_bin/SiteData.asmx
$WS.Credentials = [System.Net.CredentialCache]::DefaultNetworkCredentials
$ItemURL = "http://server/sites/promo/sales/testLib/TestFile.txt"
$GUS = $WS.GetURLSegments($ItemURL, $webID, $bID, $strListID, $strItemID)
Write-Host $strItemID

$username = "username"
$password = "password"

$cred = New-Object System.Management.Automation.PSCredential -ArgumentList @($username,(ConvertTo-SecureString -String $password -AsPlainText -Force))

#assign $cred to the webServicesProxy.Credentials
$webServicesProxy.Credentials = $cred