User nhurst on the java.net 6uN EA forum has indicated that an initially hidden applet will receive (0, 0) as its width and height when init and start are called. The old Java Plug-In set the size of such applets to the width and height originally requested in the applet tag on the page.
The forum thread describing the problem is here:
The test case has been attached. Run with the old and the new plug-ins to see the difference in behavior.
The fact that IE hands us bogus rectangles during initialization is a general problem that has come up several times during the development of the new plug-in. The "See Also" field should be updated to point at these other bugs.