Sorry to interject, but that will not make #header extend to cover the width of #wrapper - what's needed is:
There's no need for a width (or a float) - the block element will take up the full width available naturally without a width. Applying the negative margins will make the element extend the left and right beyond its normal position by 15px, negating the effect of the padding on #wrapper.
background: url("images/Star_Top.jpg") repeat scroll 0 0 transparent;
margin: 0 -15px;