Knowing that people using Windows 2000 are stuck with IE6 - despite the fact that their machine still suits their needs - is why we need to continue to push people away from IE. Even if IE9 is amazing.
Firefox 3.6 STILL works on Windows 2000 (http://www.mozilla.com/en-US/firefox/system-requirements.htm...). Until Microsoft releases a browser we know isn't going to put us in the "I can't upgrade past IE6" situations that we hear about, I'll proudly support everything that isn't Internet Explorer.
(Note: Has anyone ever tried to submit a bug report to Microsoft? It is impossible! I even put together http://crashie8.com/ as a proof of concept, and I still can't get anyone to acknowledge the problem!)