JCK: 6.0 b23
Test:
  vm/concepts/execution/execution081/execution08120m1 failed.
( The test checks that the global statics variable is unaccessible before the method <clinit> finishes )
FAILED: mustang b85 ( only on client vm - "-client" option )
PASSED: mustang b84 ( all ), tiger ( JDK5.0u7 )
Also PASSED on mustang b85 with -server OR with "-client -Xcomp" options.

PLATFORM: seems to be platform indeprndent failure
----------------------------------------------------------
Test src ( several printlns added ):
import java.io.PrintStream;

class X extends Thread {
    public static final boolean _P; // TRUE after clinit

    static {
        // _P is FALSE here
        // Run fiddle_with_X in a loop until it compiles
       for (int i=0; i < 15000; i++) {
           Y.fiddle_with_X();
       }

        // Stall for a long time to allow adverse thread to get in
        try {
            sleep(3000);
        } catch (InterruptedException ix) {
        }

        _P = true; // set to TRUE here
        System.out.println("P:" + _P);
    }
}

class Y extends Thread {

    public static boolean fiddle_with_X() {
    
        boolean xp = X._P; // Capture locally X._P
        return xp;
    }

    // What happens when we kick off a new Y?
    public void run() {
        // Sleep until X's <clinit> is underway
        try {
            sleep(1000);
        } catch (InterruptedException ix) {
        }

        boolean xp = Y.fiddle_with_X();
        if (xp == false) {
            System.out.println("Y.fiddle_with_X() returned false");
            execution08120m1.retCode = 2; //STATUS_FAILED
        } else {
            System.out.println("OK");
            execution08120m1.retCode = 0; //STATUS_PASSED
        }
    }
}

class execution08120m1 {
    static volatile int retCode = 0;

    static public void main(String[] argv) {
        System.exit(run(argv, System.out) + 95/*STATUS_TEMP*/);
    }

    public static int run(String argv[], PrintStream out) {

        // Make an adverse Y thread.
        // It sleeps a little to let X::<clinit> get underway

       Y y = new Y();
       y.start();

        // Invoke X.<clinit> It will sleep for 3000.
        // Meanwhile Y should wake-up and access X's statics.
        boolean b = X._P;
        System.out.println("P: " + b);

        if (b != true) {
            System.out.println("FAILED: Expected X._P to be true after <clinit> ");
            return 2; //STATUS_FAILED
        }
        return retCode;
    }
}

------------------------------------
------------------------------------
Test output in case of failure:
java version "1.6.0-beta2"
Java(TM) SE Runtime Environment (build 1.6.0-beta2-b85)
Java HotSpot(TM) Client VM (build 1.6.0-beta2-b85, mixed mode, sharing)

Y.fiddle_with_X() returned false
P:true
P: true
97
-----------------------------------

If we comment lines in test ( that caused JIT compilation ? ):
        // Run fiddle_with_X in a loop until it compiles
   for (int i=0; i < 15000; i++) {
        Y.fiddle_with_X();
   }
test passes.