蝈蝈俊给出的解决办法，是基于XmlNamespaceManager 的。实际上，我本人更喜欢直接在Xpath里搞定：
以蝈蝈文中的Sample为例：
string ww = @"<item rdf:about=""http://ncsp.forum.nokia.com/download/?asset_id=10357""
xmlns:rdf=""http://www.w3.org/1999/02/22-rdf-syntax-ns#""
xmlns: dc=""http://purl.org/dc/elements/1.1/""
xmlns:ncsp=""http://ncsp.forum.nokia.com/"">
<title>Advanced Audio Support v1.0</title>
<link>http://ncsp.forum.nokia.com/download/?asset_id=10357</link>
<description>Media Server Audio Streaming plug-in installation file for Nokia 9200 Series Communicators. Deprecated - use the official audio stream plug-in instead, available from SDK v1.2 and later.</description>
<dc:title xmlns: dc=""http://purl.org/dc/elements/1.1/"">Advanced Audio Support v1.0</dc:title>
<dc:identifier xmlns: dc=""http://purl.org/dc/elements/1.1/"">10357</dc:identifier>
<dc:language xmlns: dc=""http://purl.org/dc/elements/1.1/"">English</dc:language>
<ncsp:filesize xmlns:ncsp=""http://ncsp.forum.nokia.com/"">7 kB</ncsp:filesize>
<ncsp:class xmlns:ncsp=""http://ncsp.forum.nokia.com/"">Toolkit</ncsp:class>
</item>";
XmlReader read = new XmlTextReader(new StringReader(ww));
XPathDocument doc = new XPathDocument(read);
XPathNavigator nav = doc.CreateNavigator();
string strXPath = "/item/*[local-name(.) = 'identifier' and namespace-uri(.) = 'http://purl.org/dc/elements/1.1/']";
XPathExpression expr = nav.Compile(strXPath);
.......