I just installed a fresh weekly of the JDK 6 (b100) and I discovered something unusual. While compiling a demo, here is the message I got:
C:\Documents and Settings\gfx\Desktop\mailman2\mailman\src\com\sun\javaone\mailman\ui\MessagePane.java:133: warning: com.sun.java.swing.plaf.windows.WindowsLookAndFeel is Sun proprietary API and may be removed in a future release
It seems that Sun is really trying to tell us not to rely on their private API too much. I guess it makes sense with the incoming Open Source version of the JDK.