in exadel's example the Header has a bottom border. The td's only provide left/right/bottom-border. Have you checked, which classes are used by th/thead?
Thanks for you response.
But I can ensure you that IE6 has a problem with it :
As you can see, there's no border between the blue row (<thead>) and the <tbody>.
Don't you think so ?
When you see the class : .rich-table-thead
It has the following property : border-bottom: 1px solid #C0C0C0;
But it doesn't appear with IE6