I have started using two tools for testing site compatibility with IE6, DebugBar and IETester. Both are from Core Services and can be downloaed from http://www.my-debugbar.com. DebugBar is a FireBug like web inspector that works quite well, even with the CSSHandler (I have had issues with FireBugLite working in IE with the CSSHandler). IETester allows you render a website using the engines from IE5.5 and higher. They can be installed on XP, Vista, and Win7. Best of all, they are free!!!
I just thought I would share.