why won't it work...

[1Unreal]

You can't align layout elements with text-align. You have to give it a width and then use margin: auto;

.header {
	position:relative;
	background:#ffffff url('images/header.png') no-repeat;
	height:100px;
	width:900px;	
	padding:0;	
	color:#ffffff;
	margin: auto;
}