A DESCRIPTION OF THE REGRESSION :
The attached program, if executed with -Xcheck:jni under mustang-b77, leads to lots of JNI warning messages.
The same happens with the Java2D demo, if switching, for example, to the 'Clipping' or 'Colors' panes.
Note that no messages are output if the OpenGL rendering pipeline is enabled.

Environment:

mustang-b77 (Java(TM) SE Runtime Environment 1.6.0-beta2-b77)
SuSE Linux 9.2 (kernel 2.6.8-24.19)
nvidia Quadro 4 380 XGL, driver 66.29

REPRODUCIBLE TESTCASE OR STEPS TO REPRODUCE:
// javac WarningDemo.java
// java -Xcheck:jni WarningDemo
// Warnings are printed to the shell if run with -Xcheck:jni on mustang-b77.

import javax.swing.*;
import java.awt.*;
import java.awt.geom.*;

public class WarningDemo
{
    private static void createAndShowGUI()
    {
        JFrame frame = new JFrame("WarningDemo");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        
        frame.add(new JComponent()
            {
                public Dimension getMinimumSize() { return new Dimension(99,99); }
                public Dimension getPreferredSize() { return getMinimumSize(); }
                
                public void paintComponent(Graphics g)
                {
                    g.setColor(Color.yellow);

                    final Graphics2D g2 = (Graphics2D)g;
                    g2.fill(new Rectangle2D.Double(10,10,80,80));
                }
            });

        frame.pack();
        frame.setVisible(true);
    }

    public static void main(String[] args)
    {
        javax.swing.SwingUtilities.invokeLater(new Runnable()
            { public void run() { createAndShowGUI(); } });
    }
}


RELEASE LAST WORKED:
5.0 Update 6

RELEASE TEST FAILS:
mustang-b72

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
No output to the shell (as in 5.0 Update 6 as well as mustang-b76).
ACTUAL -
The following warnings are printed to the shell if run on mustang-b77.
One to three lines on startup, lots more while resizing the window):

Warning: Calling other JNI functions in the scope of Get/ReleasePrimitiveArrayCritical or Get/ReleaseStringCritical
Warning: Calling other JNI functions in the scope of Get/ReleasePrimitiveArrayCritical or Get/ReleaseStringCritical
Warning: Calling other JNI functions in the scope of Get/ReleasePrimitiveArrayCritical or Get/ReleaseStringCritical


OBSERVED APPLICATION IMPACT:
Not really serious, just hampers debugging because lots of spurious warnings are generated.