I've found couple of threads [1] discussing the exact same problem, but
I've wondered if there are some normative references governing the
change in the behavior I'm seeing between classes compiled with -target
1.4 and prior versions?

If I compile the following example with -target 1.4 and run it, I don't
get error, but If I compile it with no -target specified (or version <
1.4, using J2SDK 1.4.2) I get NullPointerException because of the
"missing" EnclosingInstanceTest instance at the given time. When I
compile with JDK 1.5 I don't get error because the default target seems
to be >= 1.4.

-----EnclosingInstanceTest.java
public class EnclosingInstanceTest {

   public static void main(String[] args) {
       EnclosingInstanceTest obj = new EnclosingInstanceTest();
       System.out.println("Start...");
       obj.doTest();
   }

   void doTest() {
       Whatever ever = new Whatever() {
           /* init */ {
               System.out.println("(2) Enclosing instance: "
                                  + EnclosingInstanceTest.this);
           }
           void doSomething() {
               System.out.println("(*) Enclosing instance: "
                                  + EnclosingInstanceTest.this);
               testInstance();
           }
       };
       ever.doSomething();
   }

   void testInstance() {
       System.out.println("O.k.");
   }

}

abstract class Whatever {

   Whatever() {
       System.out.println("(1) Super class init");
       // XXX: Uncomment the following line to test
       doSomething();
   }

   abstract void doSomething();

}
-----EnclosingInstanceTest.java--

[1] NullPointerException accessing instance member of enclosing
instance
<http://groups.google.com/group/comp.lang.java.programmer/browse_thread/thread/81
eeb5e4e0d53a33/3aafa2d47d87780a>,
   Getting a null "this"
<http://groups.google.com/group/comp.lang.java.programmer/browse_thread/thread/dd
32223ae8a25867/c2c452392f0c7d70>