SignedXML.CheckSignature() not working

[ChrisA]

I'm using Michael Gallants DecodeCertKey example to get the public key
from an X509 certificate. I then create an RSAServiceProvider and try
to use it to CheckSignature() on the signed XML file. Other sources
such as http://www.infomosaic.net/XMLSign/SecureXMLVerifyWS.htm can
verify the signature, but .Net won't. Any ideas?

Here is the code I'm using:

' Verify the signature of an XML file and return the result.
Public Shared Function VerifyXmlFile(ByVal Name As String) As
Boolean

' Create a new XML document.
Dim xmlDocument As New XmlDocument

' Format using white spaces.
xmlDocument.PreserveWhitespace = True

' Load the passed XML file into the document.
xmlDocument.Load(Name)

Dim oRSA As RSACryptoServiceProvider
' Load the X509 certificate.
Dim x509Cert As X509Certificate =
X509Certificate.CreateFromCertFile("mycert.cer")

'//Create a new instance of RSACryptoServiceProvider.
oRSA = DecodeCertKey.DecodeObject(x509Cert.GetPublicKey() )

' Create a new SignedXml object and pass it
' the XML document class.
Dim signedXml As New SignedXml

' Find the "Signature" node and create a new
' XmlNodeList object.
Dim nodeList As XmlNodeList =
xmlDocument.GetElementsByTagName("ds:Signature")

' Load the signature node.
signedXml.LoadXml(CType(nodeList(0), XmlElement))

' Check the signature and return the result.
Return signedXml.CheckSignature(oRSA)

End Function