A DESCRIPTION OF THE REGRESSION :
The following code snippet will throw an IllegalArgumentException in 1.5.0_6, but will not in 1.6 beta 2 March 9 build.

import java.awt.*;

public class test1 {
  public static void main(String[] args) {
    new Dialog((Frame)null,"Title",true);
  }
}

REPRODUCIBLE TESTCASE OR STEPS TO REPRODUCE:
Exception in thread "main" java.lang.IllegalArgumentException: null owner window
at java.awt.Window.ownedInit(Unknown Source)
at java.awt.Window.<init>(Unknown Source)
at java.awt.Dialog.<init>(Unknown Source)
at test1.main(test1.java:5)

RELEASE LAST WORKED:
5.0 Update 6

RELEASE TEST FAILS:
mustang-beta

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
In 1.5.0_6, the stack trace I receive is:

Exception in thread "main" java.lang.IllegalArgumentException: null owner window
at java.awt.Window.ownedInit(Unknown Source)
at java.awt.Window.<init>(Unknown Source)
at java.awt.Dialog.<init>(Unknown Source)
at test1.main(test1.java:5)
ACTUAL -
[no output]