10.3 Calculating widths and margins, 10.3.7 Absolutely positioned, non-replaced elements:

"The constraint that determines the used values for these elements is: 'left' + 'margin-left' + 'border-left-width' + 'padding-left' + 'width' + 'padding-right' + 'border-right-width' + 'margin-right' + 'right' = width of containing block"

IE7b2 [screenshot] takes the 100%-width of the containing block for the box, adds margin-left/-top, which causes scrollbars. IE7b2 ignores margin-right/-bottom, as if the values were over-constrained.

.box {
    top: 0;
    right: 0;
    bottom: 0;
    left: 0;    
    margin: 11em 7em 5em 3em;
    background: #03001f;