I am usually using this code
div#wrapper{width:900px; margin: 0 auto;}
div#header{float:left; width: 900px;}
div#sidebar{float:left; width: 300px;}
div#content{float:left; width:600px;}
div#footer{float:left; width: 900px; clear:both;}
and I have this markup
<div id="wrapper">
<div id="header"></div>
<div id="sidebar"></div>
<div id="content"></div>
<div id="footer"></div>
</div>
somehow the layout breaks in IE6: the whole layout floats to the left!
any knowhows on this one?
EDIT: layout is valid; centering works in IE7 and 8 (but breaks in quirks)
EDIT
here's how it looks in quirks mode