<!DOCTYPE html>_x000D_
<html>_x000D_
<head>_x000D_
<title>Center</title>_x000D_
</head>_x000D_
<body>_x000D_
_x000D_
<div style="text-align: center;">_x000D_
<div style="width: 500px; margin: 0 auto; background: #000; color: #fff;">This DIV is centered</div>_x000D_
</div>_x000D_
_x000D_
</body>_x000D_
</html>
_x000D_
Tested and worked in IE, Firefox, Chrome, Safari and Opera. I did not test IE6. The outer text-align is needed for IE. Other browsers (and IE9?) will work when you give the DIV margin (left and right) value of auto. Margin "0 auto" is a shorthand for margin "0 auto 0 auto" (top right bottom left).
Note: the text is also centered inside the inner DIV, if you want it to remain on the left side just specify text-align: left; for the inner DIV.
Edit: IE 6, 7, 8 and 9 running on the Standards Mode will work with margins set to auto.