In the InfoPath 2007 managed object model, you can reference the XML for a secondary data source from the form code-behind using the syntax:
XPathNavigator node = this.DataSources["data source name"].CreateNavigator();
For the main data source, use:
XPathNavigator node = this.MainDataSource.CreateNavigator();
The XPathNavigator instance is initially positioned to the document-level node in the data source XML.