Explorer 6 dates back to 2001. That’s almost 10 years old. Many developers as well as Google and Amazon have abandoned it and will no longer support it. But around 5% of users still use Explorer 6 and it’s a pain for all Web developers. We can’t wait for it to disappear.
I am not an expert but whether WP 3.0 will be compatible with IE 6 seems irrelevant to me. What makes a WordPress site compatible with IE6 is the theme that you will use and the functions that you will add to your theme. For example, IE6 cannot read .png images or transparency in the CSS, but there is some code that you can add to your theme to help IE6 do that. Also, sliders and special functions as Popular Tabs do not always work in IE6 and then the client has to decide whether he wants the slider or for it to work in IE6.
If your theme is incompatible with IE6, there is always some code that you can add to the CSS to make it compatible with IE6.
I hope others answer you back with different views as I’ve been following this topic for a while.