I was reading about this yesterday and according to blogs System.Xml.Linq 
has classes which resolve the problems with namespaces.

"reyes" <reyesvsn@g...> wrote in message 
news:bccce1ed-cf85-40f3-9d23-0dd6d9f8338d@v......
> Hello everybody,
>
> I have a question concerning XPath expressions and namespaces.
> Consider this XML:
>
> <?xml version="1.0" encoding="UTF-8"?>
> <newsMessage  xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
> xmlns:rtr="http://www.reuters.com/ns/2003/08/content"
> xmlns="http://iptc.org/std/nar/2006-10-01/">
> <header>
> <transmitId>LWN_2008-07-26_892</transmitId>
> <priority>4</priority>
> <channel>VID</channel>
> </header>
> <itemSet>
> <!-- ========================================================= -->
> <!-- PRIMARY ITEM : NEP -->
> <packageItem standard="NewsML-G2" standardversion="2.1"
> conformance="power"
>
> guid="tag:reuters.com,2008:newsml_WNEI6QNH" version="2" xml:lang="en">
> <!-- ========================================================= -->
> <itemMeta>
> <itemClass qcode="icls:composite"/>
> <versionCreated>2008-07-26T15:55:37Z</versionCreated>
> <fileName>20080726954SP-FRANCE-EVIAN_GOLF.xml</fileName>
> <link rel="irel:prevVersion"
>
> residref="tag:reuters.com,2008:newsml_WNEI6QNH" version="1"
>
> contenttype="application/vnd.iptc.g2.packageitem+xml"
> rtr:id="WNEI6QNH" rtr:type="idType:USN"/>
> <rtr:versionedId guid="tag:reuters.com,2008:newsml_WNEI6QNH:2"/>
> </itemMeta>
> </packageItem>
> <!-- ========================================================= -->
> <!-- PRIMARY ITEM : SNI-Text -->
> <newsItem standard="NewsML-G2" standardversion="2.1"
> conformance="power"
>
> guid="tag:reuters.com,2008:newsml_WNEI6QNI" version="2" xml:lang="en">
> <itemMeta>
> <itemClass qcode="icls:text"/>
> <versionCreated>2008-07-26T15:55:37Z</versionCreated>
> <fileName>20080726954SP-FRANCE-EVIAN_GOLF.xml</fileName>
> <link rel="irel:prevVersion"
>
> residref="tag:reuters.com,2008:newsml_WNEI6QNI" version="1"
>
> contenttype="application/vnd.iptc.g2.newsitem+xml" rtr:id="WNEI6QNI"
> rtr:type="idType:USN"/>
> <rtr:versionedId guid="tag:reuters.com,2008:newsml_WNEI6QNI:2"/>
> </itemMeta>
> </newsItem>
> <!-- ========================================================= -->
> <!-- PRIMARY ITEM : SNI -Video -->
> <newsItem standard="NewsML-G2" standardversion="2.1"
> conformance="power"
>
> guid="tag:reuters.com,2008:newsml_LWNEI6Q1CDHNJ" version="1"
> xml:lang="en">
> <itemMeta>
> <itemClass qcode="icls:video"/>
> <versionCreated>2008-07-26T15:55:37Z</versionCreated>
> <rtr:versionedId
>
> guid="tag:reuters.com,2008:newsml_LWNEI6Q1CDHNJ:1"/>
> </itemMeta>
> </newsItem>
> </itemSet>
> </newsMessage>
>
>
>
>  If the "newsMessage" node has the three namespaces (xmlns:xsi,
> xmlns:rtr & xmlns), my XPath
>
> expressions like '/newsMessage/header/priority/text()' returns
> nothing. But if I delete the third
>
> namespace (xmlns), all my XPath expressions find the nodes and return
> the text content except the
>
> expression '/newsMessage/itemSet/packageItem/itemMeta/rtr:versionedId/
> @guid' that returns the error:
>
> 'Reference to undeclared namespace prefix: 'rtr'.'
>
>  Any ideas about the problem?
>
>  Best regards
>  Reyes
>
>