CSS position absolute and full width problem

You could set both left and right property to 0. This will make the div stretch to the document width, but requires that no parent element is positioned (which is not the case, seeing as #header is position: relative;)

#site_nav_global_primary {    
    position: absolute;
    top: 0;
    left: 0;
    right: 0;
}

Demo at: http://jsfiddle.net/xWnq2/, where I removed position:relative; from #header