This is actually a bug in IE6 in quirks mode. I believe IE6 in strict mode works properly.
Technically, in CSS, all you need is margin: auto; and set a div witdh to center a div (by default, divs are blocks at 100%).
Using a table isn't the "proper" way of fixing it. You can fix this, cause of IE, by using:
That'll center it in IE6 quirks mode, IE5.5 and properly center it with Opera and Firefox. As you will see, there is a reason why developers hate IE.
yerba# rm -rf /etc