I think I just found the source for it on Google Code:
http://code.google.com/p/processing/source/browse/trunk/processing/core/src/processing/core/PNode.java
That might help...

Here is a common example, maybe someone can rewrite it for the new method.

1.  PrintWriter xmlfile;
2.     xmlfile = createWriter(cfgPath("config.xml"));
3.     processing.xml.XMLWriter writeXML = new processing.xml.XMLWriter(xmlfile) ;
4.     processing.xml.XMLElement mycfg = new processing.xml.XMLElement();
5.     mycfg.setName("MyConfig");
6.     mycfg.setString( "Background", "1");
7.     mycfg.setString( "Earthcloud", "2");
8.     mycfg.setString( "Nodemeta", "3");
9.     mycfg.setString( "Linkmeta", "4");
10.     writeXML.write(mycfg);
11.     xmlfile.flush();
12.     xmlfile.close();

Another example looks something like this...

1.  XMLElement xml = new XMLElement(this, f.toString());
2.  int numNodes = xml.getChildCount();
3.  XMLElement kid = xml.getChild(0);

See  http://wiki.processing.org/w/Changes#Upcoming_changes_in_Processing_2.0

Ah, they have done the right thing, using the Java implementation of XML processing. That's a good thing, improving encoding support, among other complains on the old one.
The dropping of P2D and P3D is a bit surprising, some speed lovers might complain... Not me, I nearly never used these modes... And indeed, supporting these alternative implementations was probably lot of work for little reward.
Overall, good things are coming up.

Ya, I've been trying to work through some of the changes on my large Processing project (over 20,000 lines of code).  Looking forward to the switch to OpenGL2, but there are things to work out.  It's not a drop in replacement.  Few libraries need some work as well.

20000 lines ... What type of project is this?

Actually, it's probably closer to 25,000 lines now.  It's a link graph program.. started out as a sketch and just grew.  Now it's a full OpenGL program built with Processing.

Here are a couple screenshots: https://forum.processing.org/topic/processing-to-jmonekyengine