Detailed IE6 information is hard to come by so far. Your question is a good one -- up to now, FORM elements have been "windowed" and not suject to z-index positioning. I believe that this means they cause another instance of the browser to activate (even though you only see one window, it's as if there are two).
Many people hope that this has changed. In IE 5.5 frames and iframes were changed to not-windowed, and for the first time they enjoyed CSS positioning. What can happen for frames may happen for forms.
Any early IE6 users out there who have the answer?